The word can seem like a sentence, and the sentence can bring a sense of helplessness, dread, despair. Dementia – the D-word – is the illness we fear the most. Many people, at the diagnosis, jump-cut forward to their anticipated end, where memories have slid away in catastrophic chunks, where language jumbles up and is lost, where chaos and loss squat in the ruins of the self. Old men and old women trapped in a disintegrating body and a failing mind…

I first see Wendy Mitchell standing at the large window of her front room, looking out. She is slender and upright, with short, peppery-brown hair and a small, neat face, and she is wearing a blue shirt that glows in the damp winter day. She smiles as I approach, a wide and undefended smile. When she opens the door, she is welcoming and although she had said in advance she was anxious about my visit, she seems relaxed and alert. The house is scrupulously tidy and clean; everything has its proper place. There’s a large diary in the kitchen whose pages are interleaved with email print-outs; there’s a wall calendar with appointments marked up in large, legible letters. I notice, as she moves about making me tea, that she has a slightly wide-legged and rolling gait, like someone standing on the deck of a ship. When she speaks, there’s an almost imperceptible blurriness to her words, as if they’re beginning to lose their hard edges and melt together. I see these things only because I know that she has been given the sentence that most of us fear.

Mitchell is 61. Three years ago, she was diagnosed with Alzheimer’s; she also has vascular dementia. She is young to have an illness that is associated with age: in the UK, about one in six people over 80 will get dementia. Yet early-onset dementia represents approximately 5% of the 850,000 people living with the condition in the UK, and it may be significantly under-diagnosed. This means that at least 43,000 people of working age have it, some in their 30s and 40s. Mitchell is cruelly unlucky but not unique.

What marks her out, however, is that she has written a book about her experience, making a narrative about the loss of narrative; finding words for the failure of language; giving a voice to emotions that are usually unspoken. Somebody I Used to Know, written with the help of the journalist Anna Wharton, is a lucid, candid and gallant portrayal of what the early stages of dementia feel like, from the days of fog and exhaustion, through the bewilderment of medical examinations and psychological tests, into the certainty and fear of knowing what was wrong – and then into fear’s aftermath, which for her meant finding a new purpose, a way to be optimistic and valuable in the world in the face of her own unravelling. This memoir, with its humour and its sense of resilience, demonstrates how the diagnosis of dementia is not a clear line that a person crosses; they are no different the day after the word is attached to their sense of confusion and the vague but insistent sense that something is wrong than they were the day before. They have an illness for which at the moment there is no cure, but they are not other; they are still one of us. It is the stigma and the loneliness surrounding the disease that turns something that is painfully hard into something that is barely endurable.

Before her dementia, Mitchell was in control of her life. She hated to ask for help from anyone (“the only person I could rely on was me: everyone else lets you down”) and she liked to plan ahead. A private, level-headed and self-possessed woman, she was the single mother of two beloved daughters now in their 30s, an NHS administrator, an organised and extremely energetic woman who would get up at five in the morning to get to the office early, who would do all the DIY in her house (she was a good painter and decorator, a nifty wallpaper hanger), and who loved outdoor exercise and wholehearted endeavour. Somebody I Used to Know opens with her running (she used to love running, being outside with the wind in her face, adrenalin surging through her). And then she falls, crashing to the ground with no explanation of how she came to be there, no pothole or bumpy ground to trip her up.

She says that she already knew something was wrong without knowing what it was: there was a fog in her brain, a thick sense of weariness, and occasionally a sudden blank – “a big dark black hole”. But it never occurred to her that this was dementia – she had “zero experience” of the illness, just thought it was what old people had, and anyway, dementia was about forgetting things, not this insidious sense of the world being skewed. The diagnosis came gradually, after a series of meetings with her GP, with a neurologist; after memory tests which she conspicuously couldn’t do, the words she was supposed to repeat wiped from her memory like chalk off a board and the clockface she drew coming out wonky and awry. She read the formal diagnosis upside down on a letter on the neurologist’s desk: some memories disappear quickly nowadays, but that one remains vivid. There it was in black and white, not just said but written.

“And I thought – well, I thought: what now? I was gobsmacked. And I felt a total emptiness. There was nothing I could do; the inevitability of what was going to happen was out of my hands. I felt very sad. And I felt sad for my daughters: you can’t escape the sadness. I think I expected the services to kick in. When you have a stroke, then you get physio. Here, there was nothing: it was just, like, a sad face, and ‘I’m sorry, nice meeting you, goodbye.’ It took me ages to get round that fact.”

It’s an illness where the whole family get the diagnosis. During the time I spent with her, Mitchell spoke repeatedly of her daughters. When she was upset, her sorrow was for them; when she thought ahead, it was their future that made her anxious. She wants to be their mother still, to protect and look after them, and is determined they should never become her carers. Two weeks after the diagnosis, she went back to the neurologist with her daughters, introduced them and left the room, so that they could ask whatever they wanted. “But you don’t know what you don’t know,” she says. You don’t know what to ask, where to turn for help; you don’t know what your life will be. “Just things getting worse.”

For six months, her life was adrift. She went to work, concealing her diagnosis, trying not to be caught out, assailed by the terrifying moments of blankness that sometimes, increasingly often, swallowed her whole. Days were dominated by trying to hold things together, by Post-it notes on which she wrote herself instructions and reminders, and by the desperate and in the end futile attempt to hold disorder and panic at bay. Things began to change only when, through the Alzheimer’s Society network, she started to meet other people like her.

“It was the first time in my life I had met people with dementia – and it was wonderful, the best thing I could have done. I saw they – we – were no different from the rest of the world; I saw it wasn’t the end, after all. There is nothing more comforting than confiding in other people like you: yes, I did that too. And no one batting an eyelid if someone thinks Queen Elizabeth is pregnant. It is absolutely non-judgmental, and oh, there’s so much laughter – because so much is very funny, the ridiculous things we can do.”

Now Mitchell takes part in research, is part of the Three Nations Dementia Working Group, talks at conferences, is a “dementia blogger” offering advice, encouragement and hope to others; wherever she goes, she speaks out about the illness. Dementia “strips away your sense of value” and of having a place and purpose in the world, but talking about it, writing about it, facing the truth, has become her life’s work for as long as she is able to do it.

Because, as she says, “never make it a secret. There should be no shame. Why feel ashamed of having a complex brain disease? People are afraid to talk about it.” When she told her work colleagues of her diagnosis, she was aware they would be shocked and embarrassed so she was as gentle and tactful as she could be, breaking the news to them and in some way comforting them for her misfortune. When her neighbour, with whom she used to chat every day, was suddenly “crossing the road to avoid me” she had to confront him and ask him why. Shame stops people facing the truth and dealing with it; it is toxic.

In her book, Mitchell intercuts the account of her dementia years with what read like wistful love-letters to her previous self – that lucky woman who didn’t even know she was lucky, who worked long hours, ran in all seasons, hung wallpaper, looked after her children, never asked for help; who, strong in body and in mind, made for herself a life that she loved. Next to this formidable stranger, she sets her frail and deteriorating present self who has come to rely on the kindness of strangers (and the love of her daughters). She has lost so much, she says: “Every day I lose something.” Precious memories fall away, recognition crumbles, her body lets her down and her mind blanks – she has a 30-minute rule for these lacunae, waiting for the shed to reappear in the garden, waiting for the knowledge of who and where she is to reform. She can’t read novels any more; she can’t bake or cook or talk on the phone. “You don’t know it’s the last time you’re doing something,” she says: last cigarette, last time you make a meal, drive a car. “My voice,” she says suddenly, staring at me with a stricken expression. “I never used to talk like this. Hesitant. I wish there was a recording of me. I sometimes feel like an alien. People who meet me think this is how I’ve always been.”

There is a gap between Mitchell’s instinctive optimism, her innate desire to be positive, and this melancholic belief that she is losing not just her sense of self, but her actual self – gradually disappearing into a dementia abyss. She admires the old Wendy, that familiar stranger, and she says of her: “I didn’t let me down. I was as good as I could be, always striving to do my best.” She is still striving, still being as good as she can be. She has made her experience of dementia into a journey of discovery. Painful loss has brought some hard-won gains: she doesn’t read novels any more but she does read poetry and shorter pieces of prose. She can’t be the unswervingly protective mother any more, but instead she can display her vulnerability and talk to her daughters about feelings she never previously disclosed. As a result, they talk in a newly intimate and self-revealing way, discussing living wills, future needs, deepest fears, and death: there’s a lot of discussion about how to have a good death. She talks of her “old self” looking after her, helping her through these hard and solemn days. She hopes that death will come before she is dismantled by the illness, or that the law will be changed to she will be able to chose her time of leaving: “We’re not this unkind to animals; we don’t let them suffer like this.”

I ask Mitchell about the shape of a normal day, and she tells me her routine, ticking off the hours. At 7.30am she gets up – whether she wants to or not. She dresses, has no breakfast but makes herself a flask of tea, and then does her brain-work to “get my batteries going”, playing online Scrabble with a friend in Wiltshire, and solitaire. At 10am, she takes the bus to the nearest town, walks through it, catches the 11 o’clock bus home. She answers emails and tweets, and plays online games again till lunch – which is just more tea. She used to love cooking and eating, but has lost her ability to prepare food and also lost her taste and her hunger.

At 2pm, she sits in the conservatory, or lies on her bed if it’s winter. Sometimes she has the radio on; usually she rests in silence. She is not thinking, not even feeling, but is “empty” and “content”: She says that she needs and craves this time, watching the birds in the field, simply being. “I wish I could do it more,” she adds. “But I mustn’t. I mustn’t!” The strict routine she has, the alarm that goes off at the same time each morning, the Scrabble, the bus rides, the regulated cups of tea, are what keep her alert. “Friends who stop have declined so quickly,” she says. “I’ve got to keep doing. It’s a scheming disease. You have to outmanoeuvre it.”

At 6pm she has supper, which is always a carton of microwaved soup into which she empties a pre-cooked packet of rice, and a slice of bread. And so to bed. As she tells me about her routine, which is like a grid that she puts over confusion and forgetting, I’m struck by the work and the willpower that go into her life now. Each day is a struggle not to give in to the dementia: to keep going, to keep doing, whatever the cost.

Does she ever feel scared? “No,” she says. “Not ever. I used to be afraid of lots of things – of dogs and cats; of the dark. Now I’m not afraid of anything.” When she thinks of the future, she knows that she neither wants to be looked after by her children, nor to be in a care home: “I’m private; I’ve never lived with other people; I know I would hate that.” I point out that one or the other might be inevitable and she gives me her sweet smile: “I know. I just have to hope I get cancer.” She dreads the thought of “going over the edge” into a world where she no longer recognises her two beloved daughters.

I ask her if she is happy in this new life she has made for herself. She ponders. “I try to do the things that make me happy,” she replies. “And I have made hundreds of new friends, all over the world. I have learnt about human nature, about acceptance, kindness, how a smile from a stranger goes such a long way. I have found a new me.” And often at night, when she looks from the silent spaces of her house, she sees a white owl beating its way across the field like a ghost. “That makes me happy,” she says. “I’m lucky.”

After two hours I leave. It is time for her emptiness and peace. She stands at the window, in her blue shirt.

• Somebody I Used to Know is published by Bloombury on 8 February (£16.99). To order a copy for £14.44 go to guardianbookshop.com or call 0330 333 6846. Free UK p&p over £10, online orders only. Phone orders min p&p of £1.99