On a hot sticky day in July 2003, Jill Anderson was driving home across the North Yorkshire moors with some groceries when something told her to put her foot down. Hurrying back, she dumped her shopping bags in the kitchen and ran upstairs, where her husband Paul was sitting on the bed. "What's wrong? Has anything happened?" she said.

"I'm sorry," said Paul. "I've taken enough this time."

He slumped on to the bed, falling into a deep sleep. By next morning, he was dead.

Nine years later, Jill is sitting in a cafe in sunny Bristol reliving that day, her marriage to "the love of her life", her arrest on charges of manslaughter – and what it was like being in a relationship with the threat of suicide constantly hanging over it.

Jill met Paul in September 1992. She was 37, and had been single for much of her late 20s and early 30s. "I'd been on countless dates, but they were never people I wanted to spend much time with, or felt comfortable with," she says. Paul was different. "I felt secure with him, felt I could say or do anything and he would accept me.

"I knew very quickly he was the one I'd been waiting for. I didn't think I'd ever meet anyone, and suddenly there he was, and it was lovely. He made life magical."

They married two and a half years later. But three days before the wedding, Paul picked up a flu-like virus. Weeks passed, then months, and he didn't get any better. He started to feel weak, and spent periods in bed. Doctors did tests, but told him there was nothing wrong, and sent him home with some aspirin. Undeterred, the couple spent hours researching Paul's symptoms online, and soon self-diagnosed chronic fatigue syndrome, or myalgic encephalitis, usually known as ME.

CFS is persistent exhaustion that doesn't go away with sleep or rest. There is no cure, but treatment can ease the symptoms, the most common of which is muscle pain. Paul was unlucky: he was one of around 25% of sufferers with severe symptoms that were compounded by health problems since he was very young, including arthritis. CFS's physical symptoms can lead to emotional and psychological problems, too, born out of frustration, and from the fact that often the medical profession doesn't take the condition seriously.

"From our wedding day on, Paul's health was up and down, but always on a downward curve," says Jill.

Shortly after they married, they moved from London to Yorkshire, for some quiet. Paul was a linguist and they had started a translation agency, which was thriving. It meant they could work from home so didn't need to live in the city. On the day they moved in, Paul "was full of beans, putting up shelves, building a dining table and bench. We were so happy, we thought he was finally on the mend. But it was the adrenalin of moving because after that he crashed."

It wasn't until three years later that a consultant finally accepted there was something wrong with Paul. He was given pain relief drugs and encouraged to seek psychological help, which he didn't do. "He was open-minded about [therapy] and would have had some if he thought it would get results," says Jill. "But he had some homeopathic treatment and fishing was a form of therapy for him."

Jill watched as her husband gradually lost his health. In the early days, he was still well enough to take a gentle stroll in the countryside. "We would do five miles, then it was four, three, two, one, until he could only walk 100 yards before he was too tired to go on," she says. His pain increased, and by the time he died, he was virtually bedridden. The business, reliant on Paul's translating, didn't survive and they were declared bankrupt.

She became his carer. "Looking after Paul, as well as trying to keep the business ticking over before it went under, was a full-time job," she says. "It got to the point when I didn't have time to even cut my own toenails."

Did it occur to her to ask for some help? "I should have done, but I wanted to do it on my own. We both thought we were coping. I was in contact with support groups, but we never had anyone come in to relieve me. Paul would get terribly upset if I even called a helpline, as he didn't want to be a burden to me. He was constantly looking for ways to make my life easier, like setting up online shopping."

A year before he died, Paul gave up his beloved fishing. He had started to get less movement in his upper back, and the pain was becoming vicious. It was around this time that he made his first suicide attempt. He left Jill a note:

"To my darling Jill, I love you more than I could ever say. I'm so sorry. I just can't stand the pain any more. Thank you for everything you have done for me. Please try to have the best life you can. I love you for ever. Your loving Paul."

Jill got him quickly to Harrogate district hospital, where they saved his life. "I understood why he wanted to die, as he was in so much pain," she says, quietly. "But it terrified me. I loved being with him and didn't want him to go."

Paul made a second suicide attempt shortly after that, this time taking a combination of pills. Again, she called an ambulance and he was rushed to hospital. From that point on, he was contemplating suicide on a regular basis – on the third occasion, a few months before he died, attaching a hosepipe to the car exhaust.

"I watched him go into the garage," says Jill. "I waited five minutes then followed him in there. I pulled the hosepipe out of the car window, and he shouted 'I want to die!' I said, you can't – and gave him a list of all the reasons to stay alive."

During those months, as suicide became part of the language of their relationship, how did she cope? "I don't know. I just saved him and then we'd carry on as if nothing had happened."

Did they ever talk about it? "Just to the extent that I would tell him how much I didn't want him to die. I became dedicated to saving his life – clean sheets, neck pillows, his favourite foods, anything I could think of to alleviate his suffering in the hope that he would make some sort of recovery, and wouldn't want to [kill himself]."

It was in the final six months of Paul's life that the couple realised his illness was serious. "Before then, neither of us wanted to admit he was terminal, that he was never going to get better [see footnote]. I lived on hope and couldn't accept he wouldn't recover. I kept thinking, what if he'd not gone out that day [when he caught the virus] and got soaked, or what if we'd moved to Devon and given him some sea air? But Paul would always say that nothing would have made any difference."

The day Paul died, he and Jill had started, finally, to fill out a detailed, 60-page form that would allow them to claim disability allowance. It was something Paul had been putting off for months. Jill now believes it was the trigger for his suicide – and it was this realisation that made her rush home that hot July day. "The form meant Paul had finally accepted that he was never going to get better," she says. "He failed on every question. It sounds stupid, but I think we were both stunned [by the realisation]."

When Jill found him, after returning home from her shopping trip, she hoped – despite what he told her – that he had not taken enough pills to kill himself. But this time, she didn't call for an ambulance. "I felt defeated. I had to accept that he didn't want to go on. I didn't want to see him pumped full of drugs again, forced to stay alive when he didn't want to be."

Instead, she left him sleeping, and lay down beside him. At 9:30am the following morning, Paul took his last breath. She called her doctor at 11am. "I wanted a few more hours with him," she writes in her memoir. "It was selfish but these moments were all I had left of him. I felt a strange euphoria; there was no physical pain in this place any more."

Later that day, the police arrived to take a statement – standard procedure. And Jill's story could have ended there, with the sad but peaceful death of her husband, an end to his suffering, and the beginning of a slow grieving process. But on 30 July 2003, nearly two weeks after Paul's death, Jill was arrested for assisted suicide and manslaughter.

She was questioned for three days – about Paul's illness, their marriage, his suicide attempts, and her actions on the day he died. She told them she didn't know why she hadn't called for an ambulance when she found him, but regretted not doing so. In September 2004, she was charged with manslaughter. The charge of assisted suicide was dropped because the police couldn't find any evidence that Jill had assisted Paul in taking his own life. She gave up her passport and had to report weekly to the police station. "It was awful. They turned my life upside down. My solicitor kept telling me, they are clutching at straws, they'll drop the charges."

They didn't. The case came to trial – by jury – on 20 April 2005. Much was made of Paul's history of hypochondria, and the fact that he had rejected psychiatric treatment.

Jill was found not guilty. Today, she believes hers was a test case, the result having serious implications for the emotive assisted suicide and right-to-die debates. Does she ever wish she'd never confessed to finding him before he died, and thereby implicating herself? After all, his previous attempts to take his own life were surely proof enough of his intentions. "No. And they would have found out anyway."

The day Jill was arrested, she told the police: "I have to live with this for the rest of my life." I ask her why, when she was so convinced that her actions were right, she said that as it suggests she blames herself for what happened.

"I was Paul's carer, and the duty of any carer is to look after someone and make sure nothing happens to them," she says. "So in that way, I failed. Because of all the media attention, and the trial, I've been made to feel guilty. But deep inside, I know I did the right thing for him. I knew he didn't want to live with such pain, feel suicidal – or try to take his own life and end up in a coma, a vegetable – permanently paralysed."

As the trial ended, Jill's grief began. "I started grieving as soon as he died, but it's been such a strange, traumatic process that sometimes I think it was only [after the trial] that I really started to come to terms with it all," she says. "And grief is not a one-off thing, it comes in waves, like acts in a play. There are certain points in your life – the trial, moving house, writing this book – that propel you forward."

Today, Jill lives in Devon, where she had planned to live with Paul. Despite her occasional sweet smile, she carries a heavy sadness about her. "Life is reasonable," she says. "But it's awful being without him. I hate being on my own. He's completely irreplaceable."

Has writing the book been cathartic? "No, it's been like rubbing salt in the wounds." She smiles. "But ultimately, I hope it will be."

Jill Anderson's memoir, Unbroken Trust: The Forbidden Goodbye of a Husband's Suicide, is published by Simon & Schuster), £12.99. To order a copy for £10.39, including free UK p&P, go to theguardian.com/bookshop or call 0330 333 6846

• This footnote was appended on 18 June 2012:
While no cure has yet been found for ME, it is extremely rare for it to be terminal and some people severely affected by ME do recover or significantly improve.