Me, myself and I: How easy is it to write confessional poetry?

"Now I come to look at love," says Sharon Olds, "in a new way, now that I know I'm not/ standing in its light." She says this in her poem "Unspeakable", in her collection Stag's Leap, which last week won the T S Eliot prize for poetry. It's just one poem in a whole book of poems which speak of the agony of lost love. The collection starts with the moment the man she has been married to for 35 years, whose love, she says, made her look "out at the world as if from inside/ a profound dwelling", tells her that his love has died.

It continues through the days, and weeks, that follow: in the conversations about "when to tell the kids", in the speech she has prepared for her mother, in the "last look" and the "last hour". And everywhere, there's the pain, and the shame. "If I pass a mirror," she says in her poem "Known to be left", "I turn away, I do not want to look at her, and she does not want to be seen." It's clear, and it's sharp, and it's forensic in its detail, and it's lyrical, and it's beautiful, and it's devastating. You can't be human and read these poems and not sometimes hear yourself gasp.

But you can't read quite a lot of Sharon Olds's poetry and not sometimes hear yourself gasp. For more than 30 years, she has been writing poetry about love, and sex, and abuse, and childbirth, and death. She has been writing not just about the emotions that go with these things, but about blood, and sweat, and semen, and how you can, for example, be trying to breastfeed your baby, and then lie on the bathroom floor in the dark, with your "bared chest against the icy tile", and slip your hand between your legs and ride "hard, against the hard floor". Except that she doesn't say "you", of course, she says "I".

Her poems are, among other things, a celebration of the human as animal. They are, as the poet and novelist Michael Ondaatje has said, "pure fire in the hands". But they are, though very, very much her own, also in a tradition. "This," said the poet Glyn Maxwell when he reviewed one of her early books, "is the sound the confessional hordes have been trying to utter since Lowell."

It was the American critic Mack Rosenthal who first used the term "confessional" about a certain kind of poetry. It was in 1959, in a review of Robert Lowell's collection Life Studies, which was about Lowell's struggles with mental illness. "Confessional poetry", said Rosenthal, is poetry that "goes beyond customary bounds of reticence or personal embarrassment". And Lowell wasn't the only poet to do it. John Berryman, in The Dream Songs, and W D Snodgrass, in Heart's Needle, and Anne Sexton, in Live or Die, and Allen Ginsberg, in Howl, were all writing about aspects of their personal experience – mental illness, the suicidal impulse, unconventional sexual desires – that poetry hadn't often covered before. But it was Life Studies which had the biggest influence. It was, said the American poet Stanley Kunitz, "probably the most influential book of modern verse since The Waste Land".

When Sylvia Plath read it, she was "excited" by the "intense breakthrough into very serious, very personal, emotional experience" which she felt had been "partly taboo". As anyone who knows anything about 20th-century poetry knows, she went on to write some "very personal" poetry of her own. In her poem "Stings", which is ostensibly about beekeeping, she has, she says, "a self to recover, a queen". Is she, she asks, dead? Is she sleeping? "Where has she been?" Now, she says, "she is flying".

It's quite hard today to imagine a culture where anything "confessional" was in any way new. It's everywhere: in newspapers, on blogs, on Twitter, on websites, on radio, on TV. You really can't get away from it. You might want to, but you can't. In books, in interviews, in columns, and in journalism, the word that leaps out, again, and again, and again, and again, is "I".

In poetry, this has led to an awful lot of what the poet Hugo Williams (who won the T S Eliot prize in 2007) has called the "I am a garden of black and red sausages" school of poetry. Anyone who has had anything to do with poetry will have seen enough of this to keep them going for quite a while. When I was running the Poetry Society, there was a steady stream: for poetry competitions, for poetry magazines, and, perhaps for my pleasure, in the post. Even here at The Independent, where we used to publish a daily poem, there was a stream: of poems, written by people who didn't know how to write poems, but who thought that what Wordsworth called "the spontaneous overflow of powerful feelings" was enough.

It isn't. "Many collections," said the poet Mimi Khalvati, when I asked her for her views, "seem to centre on a gripping issue, say the breakdown of a marriage, or the death of a partner, or infertility, but it would be a pity if this were to draw attention away from the way in which language itself is used." Khalvati, whose most recent collection, Child, tells the story of her life from early childhood, is known as one of the best poetry teachers in the country. She founded a national training school for poets called the Poetry School to help poets develop the formal skills needed to write good poems. "The novice poet," she said, "will try and express feelings they already know they have, but an experienced poet is one who knows that a poem is only a true poem if it reveals what you didn't know you felt."

The poet and novelist Lavinia Greenlaw, who's Professor of Poetry at the University of East Anglia, agrees with her. "No one wants to be called a confessional poet," she told me. "It suggests all you do is blurt your feelings. To work explicitly with the self requires extraordinary judgement, detachment and control. Sharon Olds, like Plath, has these qualities."

She does. She certainly does. Very few poets match Sharon Olds in the discipline she brings to her best work. "My job," she says in one poem in Stag's Leap, "is to eat the whole car/ of my anger, part by part, some parts/ ground down to steel-dust". The anger is there – everywhere – with the pain and the shame, but you feel it much, much more powerfully because, in the poems, it's under such tight control.

"My poetry," she said, when I interviewed her a few years ago, is "apparently personal. I've never said that the poems don't draw on personal experience, but I've never said that they do." With her latest collection, she has made it very clear that they do. She has said, in fact, that she wrote the poems when her husband left her, but promised her children she wouldn't publish them for at least 10 years. For the reader, there may be an extra thrill in knowing that the things she writes about actually happened, but the thing is, it doesn't matter. Whether or not they're literally true, they're true. "Beauty is truth," said Keats in "Ode on a Grecian Urn", and he didn't mean things that were literally true. Truth is what you find not in spilled feelings on a page, or in tearful confessions on Oprah Winfrey's sofa. Truth is what you find in art.