Lachlan Young, 23, denied he murdered Hannah McGuire, claiming her death was a spontaneous incident as he took the case to trial in the Victorian Supreme Court.
But after eight days of evidence before a jury, Young admitted he killed his former girlfriend.
Young had been controlling and abusive towards Ms McGuire and intervention orders were taken out against him, the jury was told.
After the second court order was made in March 2024, Young realised his relationship was over so he decided to kill Ms McGuire, prosecutors said.
Young told a workmate on April 2 that he would "roofie" Ms McGuire, drive her out somewhere and crash the car while she was unconscious.
Two days later, Ms McGuire went to their joint property in Ballarat's south about 9.47pm to meet with Young.
She never left the house and Young instead murdered her sometime between 1.50am and 2.53am on April 5.
He then placed her body in the footwell of her Mitsubishi Triton, drove out to Scarsdale, about 25km south of Ballarat, and set fire to the car.
As he left the scene, Young used Ms McGuire's phone to send messages to her mother Debbie, claiming she was going to take her own life.
"I'm sorry Mum, I thought this was the right decision," text messages read to the jury stated.
"I tried to heal and cope with it all but I'm not OK."
He then transferred $2000 from Ms McGuire's bank account to her mother and $5000 to himself.
Young argued at trial that Ms McGuire's death was an unplanned and spontaneous event, and while he did kill his former girlfriend, it was manslaughter rather than murder.
But for unexplained reasons, he changed his position on the eighth day of the trial and instead entered a guilty plea to the charge of murder.
Young will return to the Supreme Court in Ballarat on Monday for the start of his three-day plea hearing.
Ms McGuire's parents Debbie and Glenn, as well as other family members and friends are expected to read out victim impact statements to the court.
Young's barrister and the prosecution will then make submissions before Justice James Elliott proceeds to sentence at a later date.
1800 RESPECT (1800 737 732)
Lifeline 13 11 14
Men's Referral Service 1300 766 491
11°C