To carry a cup of tea takes concentration - lots of it - for Christine Bryden. She couldn't possibly, she says, attempt to walk holding two at the same time. "Two cups, and my feet to keep an eye on, and the step from the kitchen ... too much to think about." Nor does she cook, or drive, or go out alone.

Ten years ago Bryden was a high-flying civil servant in the office of the Australian prime minister, with a glittering career ahead. Then, aged just 46, she was given a devastating diagnosis: she had dementia. Within five years she would be seriously demented, and within eight she would be dead.

Today Bryden - who back then was a single mother with three daughters - has a new husband and her second book is just out. When she opens the door of her mother's home, in a quiet London suburb, she looks maybe slightly older than her 56 years, but otherwise nothing seems amiss. The only clue to her condition is that every movement, every sentence is careful and deliberate.

The fact that she seems so normal has caused Bryden problems: "People don't believe I have dementia. They say that if my brain scans are really mine, I shouldn't be able to speak. It's a catch-22: If you can speak, you can't have dementia so you aren't qualified to talk about it; if you can't speak, you do have dementia, but you still can't talk about it!

"I'm like a swan," she says. "You see me gliding over the surface, but in my head I am paddling frantically." Australian swans are not the white, mute variety, they are striking black birds with red bills and a loud call. Bryden has felt that, however hard she has to paddle, she must make a noise on behalf of people with dementia.

After her devastating diagnosis, Bryden contacted the Australian Alzheimer's Association for help. "Who are you caring for?" asked the voice on the phone. "Your mother? Father? Husband?" "Actually, it's me," said Bryden. The organisation had little to offer; everything was aimed at carers. "The closest I had to a carer," says Bryden, "was my oldest daughter 300km away at university." Bryden's youngest daughter, too, bumped up hard against the dementia stereotype. Aged only nine, she wrote in her school diary, "Mummy was diagnosed with Alzheimer's." The teacher called her a liar.

Bryden struggled on alone for two years, going slowly but steadily downhill and becoming more and more depressed. Finally, her can-do personality broke through and she joined an introductions agency. She didn't hold out much serious hope that anyone would want a 40-something woman with dementia, but she thought at least it might assuage her loneliness.

Her first date was Paul and they hit it off immediately. An hour and a half into their initial meeting she told him she had dementia. Paul, a down-to-earth Aussie, had fallen for Christine and, with a father who had died of Alzheimer's, he says simply, "I thought I could cope."

He has more than coped. He has taken over the practical side of life so that Bryden can spend her time using what is left of her brain to the full - a major reason, she believes, that she is still functioning. "Use it or lose it," she says, and she has certainly used it. With Paul at her side, she has travelled the world, from Japan to America, India to New Zealand, speaking about what it is like to live with dementia, advocating greater understanding and respect, and setting up dementia support groups.

Meetings of people with dementia can be amusing, she admits. In Montana (USA), she joined 10 others to set up their own organisation and online community, Dasni (Dementia Advocacy and Support Network International). "We'd have a really good creative, purposeful meeting and Paul would take notes. The next day, he'd read out the conclusions and some of us would say, 'Did we discuss that? That sounds great'."

Bryden can recall facts about dementia - that it affects some 24 million people worldwide, that one in four over-85s has it, that it affects many thousands of much younger people - but she can't remember what day it is.

She describes people she has met at conferences, but adds that face-to-face she would only immediately recognise the 10 to 20 key people in her life. She remembers furniture in her mother's home from her childhood, but can't recall what happened yesterday. It is as if the more automatic something is for most of us, the more difficult it is for her.

"After I do something, the fog closes in behind me," she says. "I have to live in the moment. I am floating in time and space. I live in a little cloud ... but clouds have gaps in them - a bit of blue sky - moments of lucidity." And what people with dementia say in those moments, Bryden stresses, "is what they really think".

In the first few years after diagnosis, Bryden was terrified that she would lose her self, a fear reflected in the title of her first book, Who Will I Be When I Die?. Not all the changes of the past 10 years have been for the worse. "I used to be quite awful to work for," she says. "I was very demanding ... I got irritated when people were slow to comprehend. And then this happened. And I've become a lot nicer ... but I feel very much still me, even though I don't function like I used to. The inner self remains." And that self will, she believes, be the one thing that stays to the end.

"The journey from diagnosis to death is a journey into the centre of self," she says. People with dementia get so little respect but they are examples of the Buddhist ideal - living completely in the moment: "As the outer layers of cognition and eventually even emotion are stripped away, we are left with the spiritual." For Bryden, her Christian faith is essential, she adds, but for others it may be nature or music that speaks directly to the inner self.

You don't have to be able to explain something in order to feel it. "If Paul makes the mistake of saying, wasn't that a great dinner last night?" Bryden says, "I'll say, what dinner? But the fact that I can't remember it doesn't mean I enjoyed it any less." And it doesn't mean it has any less effect on her subsequent mood and feeling of wellbeing.

"It really upsets me when [relatives of people with dementia] say, I don't visit any more because she doesn't remember my visits and doesn't know who I am," explains Bryden. The visit brings a good feeling even if the person can't remember why they feel good.

Paul accepts there will come a time when Bryden won't know his name. "But who is worrying about who?" he says. "That is the family's problem, not the person with dementia's." Bryden has already chosen her nursing home. "We know people who literally carry their spouses around in nappies," she says. "And we think they're nuts!" adds Paul emphatically. "It's the martyr complex," they both agree. Bryden says she would rather someone else did the practical stuff so Paul has time to attend to her as a person: "It's much more important."

Bryden has confounded her prognosis and continues to astound neurologists, but she is tired. It is time to wind down now, she says, "and accept life in the slow lane". She found finishing her latest book exhausting and does not expect to be able to speak by the end of next year. She will continue to travel, she says, but will do no more writing, conferences, talks or interviews - though Paul will.

"I have said what I have to say now."

· Dancing with Dementia by Christine Bryden, is published by Jessica Kingsley Publishers. To order a copy for £12.95 with free UK p&p, call the Guardian book service on 0870 836 0875 or go to www.theguardian.com/bookshop

How to cope with dementia

· Ask about anti-dementia drugs that may slow the decline.

· Get information and support immediately (see contacts below) - for both the diagnosed and their carers/family.

· Take photos and write things down to jog or replace memory.

· Look ahead - hard though it may be - and plan while the diagnosed person is still able to be involved.

· Help someone with dementia to do as much as possible for themselves without feeling a failure - help seamlessly; don't take over.

· If the person with early-to-middle- stage dementia gets a blank or "startled rabbit" look, they aren't coping - change what you are doing.

· Never write someone off - there is always a person in there. Even if they don't know your identity, you know theirs and that affirms their personhood.

· They may "know" you (indefinably) even if they can't label or place you.

· Don't ask questions such as, "Do you remember ... ?" It sets them up for failure or pushes them into pretending.

· Instead, describe: "It was great when ... " This offers multiple clues that may jog their memory and if it doesn't, it doesn't matter; they can just enjoy your recollection.

· Look towards the spiritual (not necessarily religion). Think what you would do - or the person with dementia might do - if you had an hour a week just to be. Listen to music, enjoy nature, stroke the cat? That may be your "spiritual".

· Just stroking someone's hand can provide comfort

· Information and support: Alzheimer's Society (for all kinds of dementia) 020-7306-0606 www.alzheimers.org.uk. Dementia Advocacy and Support Network International www.dasninternational.org