Jim Church sobbed as he told his girlfriend his life was ruined after police accused him of being unfaithful and of murdering the other woman.
In secretly recorded phone calls played at his NSW murder trial on Thursday, Church repeatedly told Belinda Lees "I have done nothing wrong" and "my life is over".
"They said 'you have been unfaithful to Belinda Lees'," he said.
"I said 'I have certainly not been unfaithful'."
While Ms Lees was supportive and loving in the calls, she broke off their relationship soon after when police showed her photos proving his infidelity.
Ms Lees continued giving evidence via video link on Thursday at the Supreme Court trial of her former partner.
James "Jim" Scott Church, 51, has pleaded not guilty to murdering Leisl Smith, who vanished from the NSW Central Coast on August 19, 2012.
The body of the 23-year-old has never been found.
The Crown alleges the then-farrier killed her after she told people she was pregnant to him and because he wanted to save his new relationship with Ms Lees.
But the defence contends Ms Smith's former partner Craig Elkin, who has since died, was a viable suspect while bikies also could have been involved.
Ms Lees told the court she believed Church was single when they began their relationship months before Ms Smith went missing.
"I would not be with a man who was sleeping with somebody else," she said.
Church told her he didn't have flings and had only been in long-term relationships.
After police showed her photos proving his infidelity, Ms Lees said she broke up with him in April 2013.
"Jim admitted having slept with Leisl and being with another woman."
She broke up with him the previous year after Ms Smith messaged her to say she was pregnant to Church.
But she said she resumed the relationship the next day after being assured it wasn't true.
Ms Lees said she was furious when Church told her police said CCTV footage captured him at Tuggerah railway station giving Ms Smith a lift, the day she disappeared.
She really "drilled" him about why he met Ms Smith and he replied she had persisted in phone calls asking for a lift.
She and Church had "had a pact we were not even going to talk to her" after she had been hassling them.
When she asked if they discussed her behaviour, Church said no and they only had "idle chit chat".
He told her he had understood he was driving Ms Smith to her home, but she got him to stop the car at Wyong and got out.
Church told her police seized his ute when a detective refused his request to keep his toolbox saying "there could be a hammer in the back you used to knock her skull in".
After Ms Smith's disappearance, Ms Lees said she thought she had a glimpse of her once in a car while Church told her of spotting her twice in a car with her father.
He said "Leisl had this coy kind of look on her face and gave a little wave".
Church sobbed throughout an intercepted phone call, telling Ms Lees police had accused him of luring Ms Smith into his car, taking her away and murdering her.
"I am going to spend all my life in jail," he said.
Ms Lees told him that Ms Smith had been preying on him and "you were her latest victim".
"I know how off the planet she is, how crazy. She is just so mental," Ms Lees said.
"It's hard to imagine until you meet the girl."
The judge-alone trial continues before Justice Elizabeth Fullerton.
