Laura Castle, 38, initially claimed Leiland-James Corkill accidentally fell off the sofa at her home in Barrow-in-Furness, Cumbria, in January 2021.
Medics treating the youngster in hospital raised the alarm though when they found his catastrophic head injuries did not match her account.
However, the defendant continued to maintain Leiland-James’s death was a tragic accident until last week when she admitted his manslaughter on the eve of her trial at Preston Crown Court.
Leiland-James was a “looked-after child” who was taken into care at birth before he was approved by authorities to live with Castle and her husband, Scott, 35, from August 2020.
The Crown say Castle was struggling within days of the placement and that text messages to her husband were later uncovered in which she referred to the baby as a “proper nob head”, a “fat shit” and a “top twat”.
Giving evidence on Tuesday, Castle told jurors that Leiland-James was “screaming” after breakfast on the morning of January 6.
She told her barrister, David McLachlan QC: “I shook my beautiful boy. I just wanted him to stop crying.”
Mr McLachlan asked: “How many times did you shake him?”
Castle replied: “I don’t know.”
Mr McLachlan went on: “How much force did you use to shake him?”
The defendant said: “I don’t know but I would say a lot.”
She explained Leiland-James hit his head on the back of the arm of the sofa and fell off her knee to the floor.
Castle said: “I picked him up. I just couldn’t take the crying. I don’t know who that person is and then it’s like I realised what had happened and I thought he was winded.
“I picked him up and his hands were like stiff, then he went floppy. He was like gasping and I just though he was winded.”
In tears, Castle accepted she had killed Leiland-James.
Mr McLachlan asked: “What went through your mind when you shook the life out of Leiland-James?”
Castle said: “Please stop crying, please stop crying. I just wanted it stop.”
She said at the time she told her husband the boy had fallen off the sofa, but omitted to inform him about the shaking.
Mr McLachlan said: “Why didn’t you tell the people trying their best to save Leiland-James’s life what had happened?”
Castle said: “I don’t have an excuse. I just panicked. I was just terrified of the consequences of my actions and ashamed.”
Her barrister said: “And police officers in interview gave you every opportunity to tell the truth?”
The defendant said: “Yes they did. I told the truth but I didn’t tell everything like I should have done.”
She admitted she had not also given a full account in her prepared defence statement ahead of the trial.
Castle said: “I found it so hard to admit that your child has died because of your actions, and I can’t take that back.”
Mr McLachlan asked: “When you shook Leiland-James, what did you intend Mrs Castle?
She replied: “I didn’t have any intention. I just lost my mind.”
Laura Castle denies murder and child cruelty. Scott Castle denies causing or allowing the death of Leiland-James and also child cruelty.
The trial continues on Wednesday