The horrifying attack took place at the family home in Scunthorpe, England back in October, 2017, after the pair were involved in a heated exchange.
Gaffney had pulled her mother's hair before plunging the seven-inch knife into her chest.
When her mother collapsed, Gaffney screamed 'I should have dug it deeper'.
The court heard that instead of helping her injured mother, Gaffney kicked her twice in the head as she lay bleeding.
She then proceeded to wipe the weapon clean and threw it out of the bathroom window.
Incredibly, the girl's mother escaped with just a 2.5cm cut on her liver.
When emergency services arrived, she refused treatment and revealed she didn't want her daughter to get in trouble.
During her trial, Gaffney pleaded guilty to wounding and was handed a suspended 18-month sentence as well as 200 hours of unpaid work.
In handing down his sentencing, the judge scolded Gaffney for her actions.
'You are very lucky, as is your mother, that more serious harm was not caused. You could have killed her, then you would have been facing a murder charge,' he said.
He added 'Your mother has had issues with drink and drugs and unsuitable relationships. That is something you resented.'
In her victim impact statement, Gaffney's mum told the court how she wanted to repair the relationship with her daughter.
'I just want to hug her. It is horrible not having contact. My life has gone downhill. I have been asked if I want a restraining order. I said no and just want to hug her. I still love her and want her to get the help she needs,' she said.