The court heard details of how the couple had split up earlier in the year and how Christison was not coping with the separation.
Justice Heath said Ms Bower had been living in fear of Christison prior to the incident after he had threatened her in text messages to their youngest son, Mathew.
In one text message, sent on February 4, about a week after Ms Bower left the family home, Christison wrote that if he couldn't have Ms Bower, no one could.
He also wrote that he was "about to lose the plot''.
The sentencing was preceded by an evidential facts hearing during which the Crown alleged Christison had murderous intent the night he killed Ms Bower.
His lawyer, Tony Snell, said he did not intend to murder his former partner that night but had instead gone to the property to scratch her car.
Giving evidence, Christison said his mother had been to visit him that afternoon and he told her he had been having thoughts of harming Ms Bower.
"I'd been having bad thoughts and bad dreams, nightmares, I have to say I wasn't that good.''
Christison said he had intended to scratch Ms Bower's car when he went over about 7.30pm.
While he was in the process of scratching the car Ms Bower exited the house through the back door.
When she realised Christison was next to the house she shouted out to him.
He responded and walked towards a gate.
Christison said he was hit over the side of the head with a piece of wood and under his throat before he "launched'' at Ms Bower.
Mr Snell asked Christison if he remembered stabbing Ms Bower two or three times.
"I do recall launching at her, I must have done it, yes that's true.''
He said he "woke up'' to find Gail on the ground.
"I realised I must have killed her and I just felt so sorry for Gail and her family, especially my boys,'' he told the court before breaking down in tears.
Christison said he cut himself then kneeled down to the ground and cuddled Ms Bower's body.
"I just wanted to go to heaven with her and die with her.''
Mr Pritchard said Christison placed a call to his mother after the attack.
"I've killed the bitch mummy, I've killed her,'' he said.
Crown prosecutor Steve Manning said Christison did not appear particularly sorry about the incident after referring to her as a bitch.
Mr Snell asked Christison if he intended to murder his former partner that day.
"No way, I love my family to bits.''
Mr Manning asked Christison why he would take a knife, leather gloves and a balaclava to the house if he was just intending to scratch the car.
Justice Heath said he was convinced Christison did have murderous intent but said he did not think Christison was lying to the court.
It was likely Christison had convinced himself of another explanation as he could not cope with what he had done.
Mathew, who discovered his mother's body, also gave evidence.
He told the court his mother had been extremely cautious with her own safety prior to her death and had been in close contact with police.
"She feared he'd do something like this.''
Mathew said his mother had installed a panic alarm, taken out the trespass and protection orders and attempted to keep their new address secret.
But his father had found out about the new house about a week after they moved in. Mathew had seen his father's BMW drive past on multiple occasions.
Justice Heath said said the offending was callous and calculated.
"The way in which the killing took place, one can only conclude, that in a colloquial sense, his soul was numb.''
