It is only when you look back, says Fiona Phillips, that you see there were signs. Her mother, Amy, was diagnosed with early onset Alzheimer's disease in her mid-60s, but her health was deteriorating long before that. "She was always quite an eccentric character," says Phillips, "which was one of the reasons we found it difficult to realise she was ill. I remember when she was in her early 50s, we went to my cousin's wedding and my mum was all over the place. She was crying, then she stood up in the middle of the speeches and her skirt fell down – she had been to the loo and forgotten to do it up. I found her in the loo crying, saying, 'What's wrong with me?'" When Phillips admits she felt embarrassed by her mother, she sounds so guilty about it.

There are 750,000 people with dementia in the UK, and the commonest type is Alzheimer's disease, for which there is no cure. Phillips had no idea that her mother had it. "I thought it could be HRT, and then she was on Tamoxifen for breast cancer and antidepressants. I thought her moods and behaviour were because of all the drugs she was on." She looks angry, remembering. "I was so furious with her doctors, thinking: what are they doing to her?"

Phillips, 49, has the same warm and chatty nature that made her a popular presenter on the ITV1 morning show GMTV. She left the programme in December 2008, after the stress of looking after her ill parents and young family became too much. Her book, Before I Forget, details the time she spent trying to take care of both. "Everyone talks about this 'sandwich generation' – people, usually women, who leave it longer to have children, so inevitably your parents are older and getting infirm." She smiles thinly. "So you're looking after babies and elderly parents, and trying to keep your career and marriage going too."

It became, she admits, too much and left her on the brink of a breakdown. "There's often one person who takes on the role of carer, whether it's because of distance or whatever. But I wanted to do it. I just felt this compulsion to be with them."

How does it feel to go from being the child looked after by your parents to the other way around? She thinks for a moment. "They were brilliant parents, but I always felt like I was supervising them a bit because they had quite a tempestuous relationship. I would worry about them and try to mediate, so it wasn't a huge shift."

Phillips and her two brothers grew up in Canterbury, where her parents owned a pub, then Southampton. Later, her parents, Amy and Phil, moved to Wales. At the time, Phillips's career was taking off – first on local radio stations in the south-east, then as a reporter on Sky News before she joined GMTV in 1993. She married Martin Frizell, GMTV's editor, and they had two sons, Nat in 1999 and Mackenzie in 2002. Shortly after Phillips gave birth to Nat, her mother came to visit, and it became obvious that something was wrong. At the coach station, Amy couldn't remember which bag was hers. Back at Phillips's house, Amy couldn't remember how to use the toaster, and she kept walking into her daughter's bedroom at night and trying to get into bed with her.

"She would cry, then laugh, then sit there with really cold eyes, staring at me," says Phillips. "I was convinced she was jealous of this new baby and how he would take all my time now. Again, I thought it was the drugs."

Before Amy became ill, says Phillips, "we were very close. She came to stay with me all the time. She was very warm – she could extract people's life stories in five minutes, and very often did. When I was a child, shopping took hours because she would speak to everybody." Now Amy cried all the time during their telephone conversations, and she would beg her daughter for help, but Phillips still had no idea what was wrong.

She remembers the phone calls to the GMTV studios from the police – one was to say that her mother had left a chip pan on which had caught fire and burned her. "I was pregnant with my second baby, I was getting up at 3am every day for work. I was beside myself." Her father was similarly struggling to cope. "He said: 'I'm ill, too.' Now I look back, he probably was."

She spent several years bundling the children into the car and driving to Wales to look after her parents. "Martin used to say, 'You've got no time for me,' and I would say, 'No, I haven't got bloody time for you.'"

It must have been a strain on their marriage. She nods. "It's a miracle he's still here. A lesser man would not have put up with all that. He used to say: 'You look after your parents and the children, and I'll look after you.' I was so bad-tempered with him, out of sheer tiredness."

Looking back, she realises she was also depressed. "I didn't want to go out anywhere. Martin would go out on his own. I just wanted to be in this cocoon and hide myself away. I used to sit on my bed, head in my hands, rocking, absolutely desperate. I remember Eamonn [Holmes, her GMTV colleague] saying to me: 'You're seriously depressed – you need help.' I used to say: 'No, I'm fine.'"

She didn't seek help, "because I knew why it was happening. It wasn't as if something was happening to me that I didn't understand. I was so tired, I had all these things to juggle and I was grieving for my mum because I was losing her day by day."

Then there was the constant guilt – for not spending enough time with her parents, for not spending enough time with her children. "I was either going off to Wales on my own or dragging the boys with me. And I was depressed and ill. I would worry that the children would look back and think I was always in bed, or always in Wales. I used to say to Mackenzie, who loves reading, 'Right, you've got 10 seconds to find a book and I'm only reading five pages.' I was always on the edge."

Amy was eventually diagnosed with Alzheimer's disease. Her condition deteriorated, and the family decided she should go into a care home. "That was a horrible decision to make," says Phillips. "I really wanted to bring Mum to London, but I was worried about her being in a strange place, away from my dad and her other relatives. She went into a home that was a five-minute drive from my dad. He didn't go and see her. Whenever I went, he didn't want to see me. Instead of thinking he was behaving oddly, I thought that the absence of Mum had exposed him for what he was. He was never a demonstrative or affectionate dad, but now he was callous and cold; we thought he didn't care about us or her or his grandchildren."

By the time Amy died three years later, in 2006, Phillips had put Phil's behaviour down to grief and regret, but it was becoming clear that his brain was not functioning properly. He had tried to make a will, but the solicitor told Phillips he was incapable. "He wouldn't want me to come to the house, so we would arrange to meet somewhere and he would get lost and not turn up," she says. "He never phoned – that just wasn't Dad – but one day I got this letter from him. It was almost like a cry for help. He was apologising for things and the writing was feathery and kept tailing off." His local pharmacist had told his doctor that Phil had been going in daily to pick up a prescription he had already collected, and the doctor called Phillips.

"I kept going to see him, and one day he let me into the house. I could see why he hadn't before. There were these really odd things – a black skeleton he had hung on his door, two brand new bikes. The house was squalid, he had been sleeping on a filthy mattress in the front room. I just said, 'We've got to get you out of here.'" She describes the realisation that her father was showing the signs of Alzheimer's disease as "like another bomb going off in our family. Just when I had my weekends back and life could take on more normality. I felt selfish really. I just thought, not again."

Phil now lives in sheltered accommodation closer to London, which makes visiting him easier. Phillips said she made the decision to leave GMTV after a particularly stressful episode. She had left work one day to visit her father and take him shopping, when the phone rang. It was the GMTV press officer who said a tabloid paper was running a story about the police picking up her father – Phil would often get lost and wander the streets. "They were going to run this story as though he was some hobo who kept getting picked up by the police, instead of a desperately ill 74-year-old man. I took Dad home and he kept following me out of the house. I was looking at my watch thinking I wasn't going to make it home to pick the children up from school. I got in the car, pulled over and had to phone a lawyer to try to stop them printing the story. Then I was late for the children."

Shortly afterwards, she and her husband took the boys on holiday. "By the end of it, I didn't want to go back to that pace of life again." She came back and resigned. She is the first to admit that she is lucky to have had the choice, even with her husband leaving his at the end of last year. She does work. "But now it's all at my own pace; I say no to things I don't want to do. I still need to juggle children, work and Dad, but it's not the same. GMTV was a big part of my life and it was great, but [leaving] was the best thing I did."

She brightens, and just before I go touches me on the arm and says she doesn't want to sound self-pitying: "I know lots of people have to go through this and far worse." Phillips says the experience of caring for her father is different – and better – than when her mother was ill. "The children are older, and I'm not up at 3am every day. Mum loved getting the most out of life every day, and to see her reduced to this weeping, mute, sometimes aggressive woman was horrible. There are heartbreaking moments with Dad – he used to be a voracious reader, but now he can only look at picture books – but he doesn't know he is ill and he is so enjoyable to be with.

"Mum was suicidal for years, but Dad seems happy – he giggles all the time. I'm probably closer to him than I've ever been because I have to do things for him – it has enabled me to be more demonstrative. I always go away from him smiling, and I know I'll have really fond memories of Dad."

Before I Forget, by Fiona Phillips, published by Preface, £18.99, is out now. To order a copy for £14.99 with free UK p&p, go to guardian/bookshop or call 0330 333 6846