A millionaire antiques dealer said “sorry” to his daughter when she woke up while he was throttling her with a dressing gown cord, a court heard.
Robert Peters, 56, strangled seven-year-old Sophia at the family home in Wimbledon, south-west London and then called 999 to report it.
He said his daughter woke up and asked him what he was doing during the attack but all he said was "just sorry", the Old Bailey heard.
Peters also explained the circumstances leading up to the killing in a video of a police interview played during his trial.
He told how he had been trapped in a two-and-a-half year long affair with a married Home Office official but denied it was the trigger for what happened.
Six months before, Peters said he had left the family home because of the fling with the woman he met online but he moved back home after it finished.
"The other party, she ended it. She had a hold over me. I tried to end the relationship a number of times but she could not let me go. I kept going back," he said.
His wife Krittiya had found out about it a year ago when she caught him on his phone.
But he added: "She forgave me, sort of, asking me questions. But that was not the real pressure. The pressure was that I was going bankrupt and unable to cope."
He said he had been "depressed" and had tried to commit suicide three times, once 20 years ago and twice more in 201
Peters did not rate himself as a "good father" and blamed his "personality" for not loving Sophia as he should.
He told officers he decided to kill her on October 3 last year because it was the last opportunity before she was due to return to her £5,000-a-term boarding school.
An officer asked: "Has Sophia given you any cause to dislike her? Is she a good child?"
Peters replied: "Yeah. She was fine. She was not good at school. Partly just her ability."
Asked if her school fees were an "annoyance", he said it was "everything - the whole scenario of my debt".
He told police he was "too tormented with my life" and "could not bear going on".
Peters said his plan was to kill his wife and family, saying: "I would have wanted to do away with all of them but couldn't."
Asked why he did not just kill himself, he said: "I can't explain."
Peters has admitted the manslaughter of Sophia but denies murder.
Additional reporting by Press Association