Stuart Hazell has pleaded guilty to the murder of schoolgirl Tia Sharp.
The 37-year-old had denied murder and sexual assault of the 12-year-old in Croydon, south London, last year.
She was last seen on 3 August at the house of Hazell's partner, Tia's grandmother, in New Addington. Her body was found in her grandmother's loft a week later.
Hazell's defence said he changed his plea because Tia's family had "suffered enough" during the trial.
He had previously claimed Tia died in an accident and he hid her body in panic.
In a statement read to the court, Tia's mother Natalie Sharp said: "I gave the ultimate trust to Stuart and there is so much that I want to ask him.
"Sometimes I feel pity, that I want to hurt him, but I can never hurt him like he has hurt me.
"She was mine and no-one else's. I had someone to love.
"Since Tia was taken I've lost my trust in everyone.
"I haven't allowed myself to grieve yet.
"When the trial is done it will be over, but it won't be for us."
The prosecution said Hazell had a sexual attraction to the schoolgirl.
The jury had been show an image of a child alleged to be Tia, dead on a bed at her grandmother's house in New Addington.
Blood could be seen on the bed linen in the picture, said to have been taken in the early hours of 3 August, when prosecutors say Tia died.
I haven't allowed myself to
Jurors heard two memory cards were found in the house, one in the kitchen and one, hidden on top of a doorframe, which contained "extensive pornography" featuring young girls.
There were also three video clips and 11 still images of Tia sleeping.
The trial was read a letter written by Hazell to his father from prison saying he had made "one mistake" and his "whole world collapsed" .
The letter was headed with the words "forgive me" and a picture of a sad face.
It read: "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."
Hazell is a convicted drug dealer and has convictions going back to 1991 for offences including burglary and grievous bodily harm.
He had a reputation for violence on the estate where he lived and three years ago he was convicted for threatening pub landlord Peter Wilson with a machete, for which he received a 12 month jail term.
"He was a horrible man, a horrible man," Mr Wilson told the BBC.
"He was a bully, not very popular at all on the estate I would say."
During the hunt for missing Tia, Hazell said she was "like my own daughter".
He described Tia as "a happy-go-lucky golden angel" and said there had been no problems at her "loving home" before she vanished.
Hazell is expected to be sentenced later.