Stuart Hazell wrote to his father whilst on remand for the murder of Tia Sharp, begging for forgiveness.
The letter was shown to the jury today in the trial in which Hazell, 37, is accused of murdering the 12-year-old.
Whilst in Belmarsh prison, Hazell wrote: "You know I'm not a bad person. It's the Hazell curse and I've only got myself to blame.
"One mistake and my whole world has collapsed. My fault, I know."
In the letter, Hazell apologised and said the death was an accident.
It goes on: "I should have told police everything. They are trying to say it was sexual. I promise you it wasn't.
"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 know Christine (Tia's grandmother) and her family will never forgive me. I'm truly truly sorry xxx."
He also wrote that he wanted to kill himself and asked God for mercy.
"God have mercy on me. Don't listen to the papers, like everyone else. I will tell you all in time. God I hate myself. I think about ending my life because if I don't someone will, that's definite," he wrote.
Hazell also asked his father to send him money, saying: "I've got no money, no fags, no hope."
Tia went missing on August 3 last year at a shopping centre in Croydon, south London.
Her body was found in the loft of her grandmother's house in nearby New Addington a week later. Hazell also lived in the house.
He denies prosecution claims that he sexually assaulted the 12-year-old, then killed her and hid the body in the loft.
The court also heard how Hazell twice drew a picture of a sad face in the letter, which was intercepted by prison authorities.
The trial, at the Old Bailey, continues.