Luay Sako, 39, has admitted murdering Celeste Manno in her Mernda home in Melbourne's northeast in the early hours of November 16, 2020.
The pair were briefly co-workers but he began harassing and stalking her after she turned him down romantically.
On the night before her death, Ms Manno posted a photo of her boyfriend for the first time on Instagram.
A few hours later, Sako broke into her bedroom while she was sleeping and killed her.
The 39-year-old has pleaded guilty in the Victorian Supreme Court to Ms Manno's murder but he's argued he only caused two of the 23 stab wounds.
Forensic psychiatrist Rajan Darjee gave evidence on Tuesday that Sako had severe personality disorder, major depressive disorder and body dysmorphic disorder.
Those conditions were crucial factors to Sako's offending and they would make his time in custody more burdensome, the psychiatrist said.
But Sako was mentally fit to plead, with the court told he tried to fake psychosis symptoms during his psychiatric assessments.
"He was exaggerating and he was trying to persuade me that he had a mental impairment defence," Dr Darjee told the court.
The forensic psychiatrist's evidence will continue on Wednesday.