Cormac McCarthy once said of writing: “Anything that doesn’t take years off your life and drive you to suicide hardly seems worth doing.” It is a quote that haunts Grace McCleen. It is also why she has not written anything since 2010, when she completed her batch of three books, and why she plans to keep it that way. In a mouse quiet voice, her Welsh accent barely audible, she explains: “Writing is really destructive to me.” She looks so fragile, I feel cruel for pummelling her with questions about The Professor of Poetry, which follows her acclaimed 2012 debut The Land of Decoration.

The new book is quieter than its predecessor. A novel about regret, loneliness, longing and fulfilment, in place of 10-year-old Judith with her miracles and models is Elizabeth Stone, in her late 50s, a respected professor of poetry, and cancer survivor. On sabbatical in Oxford to research a paper about T S Eliot’s Four Quartets, she rekindles her friendship with Edward Hunt, the man who introduced her to the poem 30 years earlier.

Written before The Land of Decoration, it cost no less to produce. McCleen was in the midst of a breakdown suffered after graduating from Oxford. I sense the hand of the therapist as she tells me McCarthy’s quote “is not a balanced thing to say”. Her voice is edged with the kind of resentment reserved for those who pinpoint our faults. Seated on the edge of a vast sofa in an office as characterless as a consulting room, the 32-year-old twists her legs. “When I was writing The Land of Decoration I was in that sort of headspace, because I didn’t see any future,” she explains.

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Rather than solace from illness, The Land of Decoration sounds like a symptom. “I completely sacrificed myself in a way.” A venetian blind taps against a window, mimicking the ticking of a clock. “I was in a kind of limbo because I didn’t see any way out from this illness.” It explains the book’s uncertain navigation between religion and mania.

McCleen’s illness was ME – “horrible beyond measure” – combined with a nervous breakdown brought on by tinnitus, a harsh condition for one as sensitive to sound as McCleen. Four Quartets resonated with McCleen’s experience. “The poem became more and more meaningful for me,” she explains as her hands dart from her lap to her fringe and back as if unsure where to put them. “I wrote my MA thesis on it in York. It was like I was writing an analysis of the tinnitus in my head. I wanted some sort of escape and the states that Eliot talks about in the poem – cessation of movement and timelessness – were what I was searching for.”

Deliverance from the edge of unreason did not come from the beliefs of her childhood. McCleen grew up in a Christian sect, of which her family remain members. When I push her to reveal which sect, she refuses to confirm more than that it is evangelical. She does acknowledge that her Welsh childhood was disrupted at age 11 by a two-year sojourn in the west of Ireland. This seems to have been a happy period. Later in the interview, she muses about her writing: “Although I might have been a lot weirder socially if I’d never gone back to school, I would have liked to have seen what I would have written if I had never gone to school.”

At school she was bullied, and in class tried to make herself invisible behind a curtain of thick auburn hair. Always the outsider and painfully shy, she drew on these memories to portray the brutal treatment of Judith in The Land of Decoration, though she insists she never had her head put down a toilet. When I mention her parents’ faith and its influence on her stories, she becomes defensive. “I do want people to see the work as standing apart from me,” she says rather snappily. “I think 90 per cent of authors draw on something in their lives whether they admit it or not. I don’t think you can write authentically without that.”

This loyalty is born from her family’s reaction to her backsliding. “They handled my loss of faith so well and were very supportive,” she insists. “They were very worried and just wanted me to be okay.” With that she shuts down the subject, so I try another tack. How has her upbringing affected her writing? “I was born into this religion so it is bound to have had a massive effect on the way that I see the world,” she says without explaining very much.

This reticence is as much a product of shyness as a desire for privacy. In all my interviews, I have never met an author speak so disparagingly about themselves: she is “weird socially” (she is not, she is quiet, but warm); she is a “terrible speaker”; she has “no social confidence”. When I point this out, she looks shocked and brushes aside my observation: “Maybe I am okay one-to-one.”

But she isn’t comfortable. Fear remains under the surface all the time with her – just as it lurks in her novels. “It is probably my most dominant emotion,” she admits. As she says it, her hands come to rest on her lap and her feet, which have been swinging back and forth, are stilled. “Someone who knows me well described me as having enough fear to run a helicopter. Something like that.” An awkward laugh, the legs swing again. “I don’t know. I am very fearful.”

Given this shyness it must have been hard publicising a book on Richard & Judy and at the Hay Festival? It was terrifying, she admits, and another reason to retire, but it was also “wonderful”. She smiles. Why? Because it meant the three books produced in so much pain were behind her. And somehow, the pain was redeemed by the public acknowledgment that in Grace McCleen we have found, as Hilary Mantel said, “a finished artist”.

Extract: The Professor of Poetry, By Grace McCleen, Sceptre £14.99

‘Professor Stone was not a vain woman but her hair was the one physical feature she liked: its mass offset her shallow forehead ... its weight suggested health and abundance in one who was slender to the point of thinness; its chestnut warmed an otherwise wan complexion, made excuses for the porcine fairness of her lashes, and added a note of colour to a person who would otherwise have appeared muted in the extreme.’