Bill Clinton has said his relationship with White House intern Monica Lewinsky came at a time when he was under huge pressure as president.
The book has had the largest first printing in history
He told the BBC that his "old demons" surfaced and led him into the affair.
The interview was recorded ahead of Monday's publication of Mr Clinton's autobiography, My Life.
"It happened at a time when I was angry, I was under stress, I was afraid I was going to lose my fight with the Republican Congress," said Mr Clinton.
"As I said, I was in this titanic fight for the future of the country, and an inevitable fight with my old demons. So I won the public fight and lost the private one."
At one point during the interview with the BBC's David Dimbleby for the Panorama programme, Mr Clinton becomes visibly angry when challenged about the sincerity of his contrition over the Lewinsky affair.
BBC Washington correspondent Justin Webb says this indicates the difference between this interview and the bland questions Mr Clinton has faced during recent interviews in the US, where former presidents are treated with a degree of reverence.
Clinton managed to salvage his family life
Mr Clinton will hold a lavish Manhattan party for 1,000 guests on Monday to mark the publication of his autobiography.
Many bookshops will be open at midnight to sell the first copies of the book, with thousands ordered in advance.
The sections about his affair with Ms Lewinsky are the most eagerly-awaited.
But the New York Times says that the book is "sloppy, self-indulgent and often eye-crossingly dull".
Mr Clinton says he has revealed more about his life than any public figure before him - probably more than anyone ever should.
In an interview with the CBS network's 60 Minutes programme, he said he now felt that the humiliation of public scandal - and a year of intensive counselling which saved his marriage with Hillary Clinton - had been "liberating".
When he first told Hillary about his affair with Monica Lewinsky, "she was angry and she was mad and she was as mad about me not telling her before", he writes.
On foreign policy, Mr Clinton says that his two greatest regrets were not cementing a Middle East between Israel and the Palestinians, and not capturing Osama Bin Laden.
The New York Times review says that much of the book is "the sound of one man prattling away, not for the reader, but for himself and some distant recording angel of history".
But interest is considerable, with Mr Clinton's publisher Knopf printing 1.5 million copies, breaking the record of 1.2 million for the first printing of Pope John Paul II's 1994 memoir Crossing the Threshold of Hope.
BBC One will be broadcasting The Clinton Interview - a Panorama Special on Tuesday 22 June at 2235 BST.