He doesn’t want to be seen as a martyr. Or as a hero. And, please, don’t paint him as a saint. John Suchet’s dreams of a happy retirement are in ruins and there is anger, guilt – even a trace of self-disgust – beneath his ever-present sense of despair.

Last week he revealed that his beautiful 67-year-old wife Bonnie, the love of his life, is suffering from dementia. During the past three years, he says, he has “gone from lover to principal carer”. And he has found that exceptionally tough.

“I have learnt things about my own character – things about which I am not impressed. There are bad moments. There are impatient moments.” He opens his hands in a gesture of helplessness. “There was a bad moment not two hours ago. And you end up hating yourself. Laden down in guilt. Because the behaviour that sends you that way is irrational and always trivial.”

The two examples he gives are exactly that: “Putting dirty plates back in the cupboard, on top of clean plates. Going into the bathroom and flushing the toilet. Not using it. Flushing it. Every minute. They build up and you snap. I am not very good in the patience stakes.”

Having plotted his career with great success, he is clearly frustrated at no longer being able even to orchestrate a day. Now 64, Suchet is an award-laden ITN journalist whose reports from all round the globe led him eventually to the position of News at Ten anchor. He left in 2004.

“I worked at my career,” he says. “I grafted at it. I made some big mistakes but I got there. Everything in my life I have ever done, I have done out of free will. You either succeed or fail. And now . . . this is something which has hit me – us – and I have no control over it.

“If you are reporting on a story, you need to be in control of it. As a newscaster, you need to be in control. I am not in control of this. It has come from outside. And it is bloody annoying, because I can’t do anything about it.”

Last year he was given a lifetime achievement gong by the Royal Tele-vision Society. Bonnie went with him to the ceremony – the last time they were seen together in public. He doesn’t take her out to events now; they are too confusing for her. Looking after her has become not dissimilar to tending to the needs of a small child – or, rather, a child whose life is regressing.

“Bonnie’s world is like this,” says Suchet, putting his index fingers and thumbs together to form a circle. “And it is slowly contracting,” he continues, closing the circle. “I have to step into that world with her. And if I try to take her out of it, she is confused and lost.

“She looks to me for her every mood. If I’m happy, she’s happy. If I’m angry, she’s angry. When I come back home tonight, I can say I had a lovely time with this Sunday Times reporter and she will smile and say, ‘Oh, good’. She won’t say, ‘Where? Why? Who was she? What did she ask you?’,” he says, using the reporter’s lexicon.

He pushes back his hair. “I’m losing the woman I love and have lived with for nearly 30 years. That’s worse than physical pain. It’s . . .” He shakes his head.

Does the fact that he witnessed pain and despair in his working life help him to accept what has happen-ed? “Most of the jobs I did [there’d be] poverty all around you but you’d stay in the luxury Hilton. In India I’d be at the New Delhi hotel or something, with gold-plated banisters, and beggars [would be] 10 yards away outside.” He shrugs: “You get on a plane and come home. Nothing prepares you for this.”

Two things help him. The first is the assistance of Ian Weatherhead, one of only 60 psychiatric nurses in the country whose specific role is to counsel and advise people in Suchet’s position: “I go and see him once a month or fortnight. I can e-mail him or call him any time. He helps me understand why Bonnie is behaving as she is. And, more importantly, he can help me understand why I behave the way I do.”

One of the reasons why Suchet has gone public about Bonnie is to draw attention to the need for more specialist nurses. His other great prop is the music of Beethoven. A gifted musician himself, Suchet is writing his sixth book about the composer, having turned a passion for his music into something of a second career.

When we talk about Beethoven, Suchet’s face relaxes, his eyes become briefly merry and he leans forward with enthusiasm. He dives into his coat and pulls out an iPhone – “my leaving present from Five News” (where he worked briefly after ITN). The menu is stacked with Beethoven. “A bit one-tracked, aren’t I?” he jokes. “I can blast his music into my head whenever I want – and frequently do. Beethoven’s music is inspiring; his harmonies make you want to get up and punch the air in triumph.”

A picture of Bonnie flashes up on the screen. Taken about 10 months ago, it shows her smiling and looking utterly normal. “I wish the diagnosis had been cancer,” says Suchet. “If she had cancer, we would be fighting it together.”

Their 24-year marriage had been exceptionally happy; both had been married before – she had two children and he had three – but both felt they had finally met their other half. Suchet tells me with pleasure that grandchildren are “arriving all the time” and he has an open invitation from the children to “have babies climbing all over him”. Yet it is abundantly clear that he feels looking after Bonnie is a lonely job.

“I am on my own and this is what this disease does. Ian tells me I must make room for myself. And I do. I have been away. I take the waters at Baden-Baden and go to an opera. On my own. Everything we have ever done before, we did together.”

Is he bitter that he has been denied a contented old age? “Just slightly. If I were religious I could blame God.” His great fear is that he will go first. He tells me with certainty that he will not be around in 2027 – the 200th anniversary of Beethoven’s death. He believes he has been fatally sapped by the stress of caring for his wife.

“I know where I’ll be in 20 years’ time: brown bread. No question. The question is where I will be in 20 months’ time. Seriously. It’s very, very stressful. This stress is a killer. Maybe next week, next month, or maybe next year – in a blazing flash of light, Beethoven blaring in my ears, I will go zip. And Bonnie, bless her, will go on for another 20 years.” Since he went public about Bonnie’s disease, Suchet has had a sack-load of letters of sympathy from friends. “If she had died,” he says, “the letters would have been the same.” He was reading one of these letters last week, tears rolling down his face, when Bonnie walked in. “I managed to turn my face into a smile. She smiled back and walked out again. Little did she know what I was doing. This is the woman I adored.”

Sometimes he has “good” days. “Even on a good day, I suddenly feel the tears coming and go into a little room on my own and have a little howl. I ask Ian why and he says, ‘Because you are going through the grieving process. You are in mourning’. And I thought, he’s right. I have lost her.”

Suchet thinks now that the disease must have taken hold when Bonnie was about 61 and started becoming unusually forgetful. “Hundreds of people have e-mailed me to tell me how dementia affected them, but it’s always about their parents or grandparents. Try to imagine what it is like when it hits your life partner.”

As we are talking, his phone rings. His brother, the actor David Suchet, is calling from a dressing-room at the Old Vic to ask him out for supper this week. Suchet is delighted; he admits he can “howl” in front of David and his other younger brother, Peter, more easily than he can with friends.

Does he sometimes secretly wish that his wife would die peacefully, but soon? “Oh, don’t ask me that. Even to think of Bonnie dying is too brutal. But deep in my heart I want her to be spared the ravages that are down the road.”

It’s impossible to be sure, he says, exactly how the disease will progress: “What I have learnt about dementia is how little the doctors know. They don’t know what causes it, how to stop it coming, how to treat it, how long it’s going to last.” He can’t even say with certainty that Bonnie has Alzheimer’s disease: “Alzheimer’s is just the most common form [ of dementia], but you cannot diagnose it until after death.”

It doesn’t really matter, anyway. What matters now is that Suchet is living with someone who needs him, but who in all other ways has left him.

He cannot share photographs with her or treasured memories. There is no point in talking to her about the trip they took together on the Orient Express for their 20th anniversary because she cannot remember it. They no longer have any shared interests because she cannot recall them either.

“Actually, that’s the only goodish thing in all this – that she doesn’t know,” says Suchet. “I know the past is a closed book, but she doesn’t.”

Normal conversation about ideas or plans is impossible. “One thing you mustn’t do is make a sentence with more than one idea in it. Once, about a year ago, I gave her a sentence with two, or maybe three ideas in it, and she sighed and said, ‘Oh, I don’t know. I am Bonnie@confused.com’.”

It was a rare flash of insight. “I nearly cried with joy that she was able to make that joke,” says Suchet. He is extremely protective of her feelings, shielding her from his anguish and doing all he can to lessen her confusion.

“If her son Hereward rings up, I bring her in and say, ‘It’s H on the phone, darling – say hello to H’. I never give her the phone without telling her who it is.” Does she know who her children are? “Sometimes. Sometimes not.”

He looks at me sadly: “I know I’ll be known as the man whose wife had Alzheimer’s. Not as a war reporter, or newscaster, or Beethoven scholar.”

He shrugs: “I am very single-minded, very ambitious, very focused. All those things count for nothing in the face of this.”

John Suchet supports the charity For Dementia and also Admiral Nurses, specialist nurses who work with families living with dementia.