This is the voice of experience, for she is a woman who has had more than her fair share of bad days. Indeed, misery and fear knocked on her door almost four years ago, and have since invaded her life like uninvited guests, almost tearing her beloved family apart and creating rifts that may never be healed.

The mother of the troubled rock star Pete Doherty ("Britain's most notorious drug addict," she sighs wearily), she had arrived in London from her Dorset home several hours too early for our afternoon meeting because she was so nervous about it. At the station bookstall she found photographs of her son, with "irresistible love" Kate Moss, plastered all over the cover of Hello! magazine - and the words, "Plus Jackie Doherty: a moving interview about her son".

Jackie says she knew nothing about this. She is a respectable woman, a nurse, an army wife, the mother of three children and a dedicated reader of the genteel The Lady and the poems of Pam Ayres and Patience Strong, whom she believes offers heartfelt sympathy for those whose lives are broken. She has no truck with sensationalist, gossipy publications, but she had given an interview to an English daily newspaper as part of a deal to publish extracts from Pete Doherty: My Prodigal Son, the heartrending, loving and sometimes amusing book she has written about her "wonderful, clever, happy, funny" boy and the effect his descent into debilitating drug addiction has had on her family.

Jackie insists that she was not aware that the interview would be syndicated and it has embarrassed and upset her. The interview with me - for which she is not being paid - is the only other print interview she will give. "When I saw that magazine cover, I almost got back on the train. And I've been back to Waterloo station twice this morning because I was so distraught," she says, her eyes brimming with tears. "I thought, 'I'll just go home.' But I had given you my word, so here I am." She gratefully takes a seat on a vast sofa in the hotel where we meet.

For the next 90 minutes, 53-year-old Jackie's hazel-brown eyes - her cherubic-featured son is the image of her, although she says she can't see it, joking, "I have no tattoos" - often fill with tears. "I'm a crier," she admits. She explains that crying is good for you, something she knows from her 34 years as a nurse. "It's like a soul washing." Weeping, often for days and nights on end, has helped her to cope with pain and anguish, the shame of having a son who is both a convicted criminal and a heroin addict. Her husband Peter's response has been different - he has vowed not to see his son until he is clean, and has barred him from the family home.

Peter senior is a major in the Royal Corps of Signals, and over the years his postings have taken the family to several countries, including Germany, Cyprus, Northern Ireland and the Netherlands. Their daughter AmyJo, now 29, is a teacher, while 19-year-old Emily is also in the army. Pete was born in Hexham, Northumberland, when the family were stationed at Ouston.

What Jackie describes as "the Peter problem" - he'll always be Peter to her, never Pete - first came to light on the day of her mother's funeral, in April 2003, at which a fidgety, tearful, melancholy Pete was a pall-bearer. His behaviour led to his parents sleeping in separate rooms for about a week, because Jackie refused to switch off her mobile phone. "I have to be contactable at all times. Peter often rings me at 3am, so there is no peace in the house."

Tomorrow the Peter problem will be back in the headlines again, because the gifted songwriter and former lead singer of The Libertines, now heading Babyshambles, is due to reappear in court. He is to be sentenced after pleading guilty to possessing heroin, crack cocaine and cannabis earlier this month. And all this amid a flurry of tabloid speculation that he and supermodel Kate Moss have resumed their dangerous liaison, are secretly engaged and plan to marry soon.

On August 18, the day before his mother's story was due to be serialised, her 27-year-old son pleaded guilty at Thames Magistrates' Court to five counts of possessing class-A drugs. On April 20, hours after he had been given a two-year community sentence for previous drug offences, he was found in possession of heroin, cocaine, crack and cannabis. After searching his home in east London later that day, police found more cannabis. The court was told last month that on August 7 - four days after failing to answer bail - he was again found to have drugs in his possession, including crack and a crack pipe. He is also awaiting sentencing for a further two class-A drug offences. He was released on bail on August 18, "not without some reservations" on behalf of the court, on the condition that he attend London's Priory clinic and obey a 10pm-to-8am curfew until sentencing. He was warned that he could face a custodial sentence when he returns to court tomorrow.

His mother has not yet decided what she will do. She used to go to court with Pete's sisters, "showing a united front". Sometimes, they have turned up incognito, "disguised with the help of wigs and other accessories". Of course she wants to be there tomorrow for her son, for whom her love has never wavered - despite accusations of being "an enabler" by not cutting him off in the name of tough love. "I can't turn my back on him - I'm his mother and I love my son. I'll never walk away from him, because he's sensitive and vulnerable," she says, although her heart aches for life as it used to be. "I am sick and tired of being sick and tired," she writes.

If she turns up at court tomorrow, she knows it will only add to the media frenzy. "If I am not there, I'm condemned as not caring about Peter." Damned if she does, damned if she doesn't.

"I don't know what to do," she despairs. She fears that if her son gets another jail sentence (on her 50th birthday, in September 2003, he was sentenced to six months in prison for breaking into a flat belonging to Carl Barat, his Libertines bandmate and close friend), it will only exacerbate his illness. "Peter is sick, very sick, like thousands of other drug addicts," she says, adding that she threatens him, with typical Liverpudlian directness, that when he's better she'll batter him.

In all the agonising over why this nightmare happened to her family, Jackie always tells Pete, "I blame the parents," to which he replies, "If you must, then blame them for the good things too." But there aren't any good things any more, Jackie writes forlornly.

"I have to hang on to my sense of humour, though," she tells me, adding that her grandson, three-year-old Astile, Pete's son by the singer Lisa Morrish, is a constant source of joy and laughter. Recently, Astile came to stay with his grandparents in Dorset for a week. She says he is the spitting image of Pete, who has limited access to the child, but he has managed to put the smile back on her husband's face. Peter even calls him by the same pet names he used for his own son, Billy Bilo and His Lordship.

She has not seen her son for about six weeks. And although her phone is on day and night, she hasn't spoken to him since he checked in to the Priory on August 20 for what is his seventh round of rehab. He has repeatedly fled from various clinics, and has had two implants (to suppress the effects of heroin) inserted in his stomach. "It is heartbreaking," she agrees, but then she adds that she'll tell me what she always tells her husband. "A mother's heart doesn't break - it's made of stronger, sterner stuff."

As for the timing of this latest twist in the sorry saga of her son's self-destructive lifestyle, she simply says, "A friend said to me, 'God's timing is perfect,' and I have to believe in that."

Jackie has often been described as a religious woman. "I'm not," she says, sipping black coffee. "I'm a realist. But I do have faith. So faithful, yes. Religious, no. Faith is from God. Religions are man-made."

Her devout, non-denominational Christian beliefs have helped her to survive as she watches helplessly while her son ruins his life. "Peter is a hedonist," she says flatly. Her worst fear is that he will die. She writes in her book, "I meet Peter's fans, who say they never miss one of his gigs in case it's his last. But I know he doesn't want to die; he enjoys life too much."

The parable of the prodigal son is the leitmotif of her book. She recalls that the father did not bind his son to him, but waited for the repentant to return of his own free will, and she repeats the story again as we talk. She is clearly hoping against hope that the child who used to love playing football and cricket, swimming, riding and abseiling, and who got outstanding reports at school, will eventually return to the fold, despite writing what appears to be the longest suicide note in rock history. "I can't kidnap him and lock him up, although there have been times when I have wanted to. Friends tell me to take him to the Scottish Highlands for six months or a year, but Peter is a grown man; he is 27. And I never tied him to my apron strings anyway. If I did kidnap him and lock him away, I would lose him."

Jackie has said to Pete, and to her colleagues at the hospital where she works, that a dose of insulin would put the Peter problem to sleep for ever. "It would kill him," she writes. "What an awful thing to think. I'm not proud of thinking it. I'm ashamed, but that's how I feel at times. And I think all mothers of addictive children must feel like this."

Elegantly dressed in a smart, black outfit, accessorised with 1960s-style jewellery, all bought from charity or second-hand clothes shops, Jackie is perfectly groomed, her fingernails painted the same shade of scarlet as the toes peeping out of her gold mules. On her head she wears a pert black beret - she collects hats, which may account for Pete's stylish taste in titfers - and her thick pewter-grey hair is styled in a chic bob. "My two brothers say to me, 'Wacky' - their nickname for me - 'why don't you die your hair? It would take years off you.' But I'm too mean, it's my Jewish blood," she says, referring to her father, who was the son of a French Jew and a Russian mother. "Peter was so close to his grandad, who would be heartbroken to see him now," she says.

She is a very private person, she insists. "So why have I done this book?" she asks before I do. "To help other mothers, the heartbroken families of other addicts, who have told me awful stories. They have no voice and for some reason people are queuing up to hear mine, although I feel I have nothing to say. As the parent of an addict, you feel only a terrible burden of guilt and shame, but Peter has never brought trouble to our door, never stolen from us, never beaten us, as other junkies - how I hate that word! - have done to their families. It's the press that brought trouble to us. If my book helps one other person, it won't have been in vain.

"Already, it has done that," she continues. "It has helped my husband immeasurably. In the Doherty home there has been much sadness, marked at times with a deafeningly silent anger. But recently we spent hours looking through stacks of family snapshots, marvelling at Peter's old school reports and academic achievements, recalling so many great times, reminding ourselves of the bright, happy person he used to be. Happy, happy days. What a waste."

She insist that the money she makes from the book will be saved in case Pete needs medical help. "He may end up very ill or mentally impaired, and he will need to be cared for," she says. "He lives the good life now, so he may also end up penniless - although, like me, he has never cared about money or possessions, just so long as he can stand up and sing."

A well-known Scottish actor of my acquaintance knows Pete Doherty well, after introducing poetry evenings with him at a London pub. "I really like him - he's a true romantic, a real poet," he tells me. "What people don't realise is what a big guy Pete is - not just physically, although he's 6ft 3in tall and nothing like as frail as the paparazzi pictures make him out to be - and how strong he is, mentally. He'll survive all this; I know he will. He's such a talent."

Then he tells me, with a grin, about the nights he spent with Pete in the rock star's squalid flat, which was filled with broken furniture, drug detritus and the infamous pictures painted in Pete's own blood. "I went all the way on drugs with Pete - doing cocaine, heroin, the lot," he confides. With friends like these, no wonder Doherty is in such a mess.

It's a view echoed by his mother when I tell her this story. "The trouble is," she says, "people love Peter. He's so enigmatic, so charismatic, they want to be around him."

There is always an entourage swirling in his wake - other musicians, adoring fans, unscrupulous hacks, hangers-on. Many of these people are seedy, but others mean well, she maintains. Some sell stories about him, however, even contriving pictures - like those in April of him apparently injecting an unconscious girl with heroin. So she has not shown Pete her book, lest it fall into the wrong hands, although she knows he'll love it. "He's always telling me, 'Mum, you're a brilliant writer,' which isn't true - although he is - but he did encourage me to write poetry and to publish my poems, Time for a Rhyme."

And what about Kate Moss? The model's glamorous cachet has grown along with her bank balance, despite last year's infamous footage of the couple supposedly snorting drugs in a recording studio. Jackie presses her lips together and gently shakes her head from side to side, indicating that this is a no-go area. She refuses to discuss her son's attraction to Moss, whom she is said to have met only once. In her previous interview, she was quoted as saying, "I think everyone should leave them alone. They've been deeply in love and people just won't leave them alone." When I mention this, Jackie denies it. "What I actually said was that Peter was deeply in love, and I wish people would leave him alone."

Before we part, Jackie repeats something she has written in her book. "Whenever I think the Peter problem can't get any worse, it usually does." I quote Shakespeare's words in King Lear. "This is not the worst so long as we live to say it is the worst." She gasps. "Oh, that's so beautiful. Thank you. I'll always remember that." And again her eye blur with tears.

Jackie was not a depressing interviewee; she was an inspiring one. "I've just realised that I never gave you a Latin phrase when we met," she tells me on the phone. Every chapter of her book begins with a Latin tag because, "despite not being the brightest button in the box", her love of them is well known. "Dum spiro spera - while I breathe, I hope," she says.

Certainly, while she breathes, prays, waits and loves, there is hope for her prodigal son.

• Pete Doherty: My Prodigal Son, by Jacqueline Doherty (Headline, £16.99), is published next Monday