The man on trial for murdering Tia Sharp wrote to his father from prison claiming that her death was an accident and begging for forgiveness, a court has heard.
Stuart Hazell, 37, wrote to his father Keith from Belmarsh prison in August last year, saying he had made "one mistake and my whole world has collapsed".
The letter, read to jurors at the Old Bailey on Friday, was topped with the words "forgive me" and a sad face.
Hazell, who is accused of murdering Tia, is said by prosecutors to have sexually assaulted the 12-year-old then killed her and put her body in the loft of the home that he shared with her grandmother in south London.
He denies the killing, claiming that Tia died in an accident and he panicked and hid her body.
Hazell wrote: "What happened I will explain in time, but put it this way, it was an accident and I panicked. Stupid I know but for my stupidity I'm looking at 15 to 18 years. I regret it every second of every day.
"I think about ending my life because if I don't someone will, that's definite."
Tia Sharp was murdered in August last year
The letter went on: "God I hate myself. I should have gone about this a different way, told the police everything. They're trying to say it was sexual but I promise you it wasn't."
Hazell also asked his father to send money to the prison, saying: "I've got no money, no fags, no hope. It's the Hazell curse and I've only got myself to blame."
He said he had made: "One mistake and my whole world has collapsed. My own fault I know.
"Don't listen to the papers, like everyone else. I will tell you all in time."
The window cleaner wrote that he was "truly, truly sorry", and went on: "May God have mercy on my soul even though I don't deserve it."
Earlier the court heard how Hazell, 37, had gone around asking for help looking for Tia on 10 August, not realising that the police were hunting him over her death.
He was seen in a CCTV clip in a convenience store in Morden, south London telling the shopkeeper and a young girl that he was looking for Tia, and later on Cannon Hill Common.
Witnesses said that he appeared drunk, the Old Bailey heard, and shopkeeper Prasanna Jayakumar said he looked "emotional" when he came into her shop twice to buy a half-bottle of vodka and a lighter both times.
In a statement read to the court, she said that Hazell "looked as if he was struggling to cope".
Hazell said he was looking for his granddaughter.
"He was asking me to help him find her, and if I saw or heard anything to tell him and let him know," the shopkeeper said.
Prosecutors say Tia died on the night of 2/3 August, but her body was not found until 10 August. Police then began looking for Hazell.
Hazell also approached a young girl in the shop, who recognised him as the man police were looking for after seeing him on the news.
Dog walker Victoria Dragnet, who had gone to Cannon Hill Common to help look for her friend's dog, asked Hazell if he had seen the animal.
She said Hazell told her: "I've got more important f****** things to do than look for a f****** dog. I'm trying to look for my granddaughter, she's gone missing."
She said: "I thought that his eyes were glassy and he seemed to be high, in fact he looked like he was off his face.
"He was rough in appearance and rough in his manner, he made me feel uncomfortable to the point that I just wanted to get away."
She said Hazell "gave me the creeps" and struck her as "a volatile man".
The trial continues.
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