Maxine Benson knifed 18-year-old Alfie Stone to death after he and his brother Jake were accused of pinching food from the kitchen at a block of bedsits.
The 32-year-old, of no fixed abode, denied murder but she was found guilty by a jury at the Old Bailey on Tuesday (April 26) .
Alfie desperately tried to use a chip shop advertising board as a shield, but died hours after the attack in Ickenham High Street, near West Ruislip station, on November 9 last year.
Judge Richard Marks QC described his death as a "senseless, mindless killing".
'I see the heartbreak in his mother's eyes'
Benson said she "forgot" the knife was in her hand but she was convicted of murder by an Old Bailey jury.
In a letter to the judge, Benson wrote: "I want to say how devastated I am for the Stone family, I don't want you to have mercy on me. I feel I deserve to be dead.
"I see Alfie's mum every day in court, I see the heartbreak in her eyes and know what it's like to lose a child."
See the powerful letter in full here.
The prosecution claim Benson's friends Steven Hawgood, 28, and 29-year-old Corine Cripps egged her on by shouting "stab 'im, stab 'im".
Alfie and Jake knew the area well because their sister Shannon was a tenant in one of the bedsits, although she had moved out a few weeks before the attack.
Two days before the murder, Jake had stayed at the address following a fight with his mother, and allegedly took the food from the communal kitchen.
The fight broke out when the boys came to try and collect some of their sister's possessions and Ms Cripps - known as Coral - tried to throw them out, telling them: "this isn't a doss house".
Giving evidence, Benson said she had picked up a knife to scare the boys after the row broke out. But instead of using it to threaten them, she had hidden it up her sleeve and only pulled it out at the last moment.
Alfie suffered four knife wounds to the chest - with the final blow sinking 10cm into his chest cavity to his heart.
Benson said: "I'm sorry he's dead. I didn't mean to do it. It was an accident."
'The red mist had well and truly descended'
Prior to the attack she had taken her two children - who both live with her mother in Paddington - to school. She had spent the rest of the day drinking and smoking with friends in her bedsit before the argument broke out.
Jailing Benson for 23-and-a-half years, Judge Marks said: "In your case, the red mist had well and truly descended so far as you were concerned.
"You passed into the alleyway and then into the street. Alfie can be seen on CCTV footage backing away from you in an attempt to repel an act by you.
"He picked up the wooden board outside the fish and chip shop and threw it at you.
"In my judgement, given you're much littler than him, this only could have occurred because you were in possession of that knife.
"This was in any view a senseless, mindless killing of a boy who was only 18 years of age who had the rest of his life ahead of him."
Benson wrote in her letter to the judge: "I wish I could replace Alfie's life with mine, I love my daughter with all my heart and when my daughter cries and says she misses me I know what I have done to Alfie's family.
"My cousin was stabbed to death two days after Alfie died and his killers still haven't been caught.
"I feel God has taken a life from me because of the life I took - not a day goes by when I don't truly regret what I've done."
Benson has 17 convictions for 33 previous offences, including robbery, possession of an offensive weapon and assault occasioning actual bodily harm.
Benson; Mr Hawgood, of Kingston Avenue, West Drayton; and Ms Cripps, of High Road, Ickenham, all denied murder but Benson pleaded guilty to manslaughter.
Ms Cripps and Mr Hawgood were both cleared of murder and the jury were unable to agree on a charge of manslaughter. They will learn if they will face retrial on Friday.