The Oscar-nominated star spent a significant portion of her upbringing in the care system.
The 43-year-old previously revealed she was charged with attempted murder at 14.
She was convicted after threatening to kill an older girl with a knife who was bullying her in the care home.
Remembering the incident during an appearance on BBC's Desert Island Discs, she said she attended a rave, had taken drugs, and came home to find a nine-year-old boy had been "pimped".
The BAFTA and Golden Globe winner said she "snapped" - suspecting the girl was guilty.
She said: "I snapped and said I was going to kill her. I didn't harm her, I didn't touch her but I said those words. And I regret it and I am sorry and I was sorry to her."
The mother-of-three, whose roles include Oscar-nominated Woody Allen's 1999 comedy Sweet and Lowdown and the 2003 drama America, said she spent three days in solitary confinement - in an adult cell.
"I was mortified," she said.
She added: "And I'm sorry to her. We were all abused. She was a child herself. Nobody looked after us properly. We were rioting in that home because they were locking the fridges at night.
"We were not safe."
Morton also said: "I am incredibly sorry. But it wasn't just like self-defence. I was angry at the system." She said she wanted to "put the record straight a little bit".
Morton said her strong faith in God helped her get through the testing periods in her life.
She said: "When I was little I did get very confused with my relationship with faith but I always felt loved.
"And in the same ways I look at my children and I know I love them, and it's so enormous and it's so overwhelming and it's so huge, that's the love I feel from God."
Morton went on to say: "And so when you're small or if you're in pain or in a very, very, tough situation, by accepting that love and allowing that love, the most amazing, transformative things can happen to you.
"So that's how I think that I was able to survive, I suppose."