The mother of two children who were stabbed to death in 2009 testified against the accused -- her ex-husband -- in court Monday.

Isabelle Gaston testified at a St-Jerome courthouse against former cardiologist Guy Turcotte.

Turcotte is facing two counts of first-degree murder in the deaths of his five-year-old son and three-year-old daughter in 2009.

Emotions were high for Gaston at the beginning of her testimony, choking back tears as she talked about her ten-year relationship with Turcotte. She described it as having many ups and downs and at least two separations.

Gaston testified that she confronted her husband when she discovered gay porn on his computer, saying that he told her she was crazy, and it was in her head. She also talked about her affair with her personal trainer and how she profoundly regrets it, but added that her new lover helped her see that she could be loved differently and respectfully.

Gaston spoke about how Turcotte found out about her affair on family trip to Mexico a little less than month before the murders and left the house when they returned home.

She said that on the night before the murders she and Turcotte had an argument over the phone because he did not want Gaston’s boyfriend around their children.

 She called her lover to come over, and the next morning, Turcotte came into the house and yelled at her, asking what the man was doing there.

She decided to change the locks and that afternoon as she was heading away for a girls’ weekend. She said she had a phone conversation with Turcotte where she told eventually told him she'd changed the locks. She said he was angry and yelling and she felt threatened to the point that she cancelled her joint account and joint credit card and was thinking of heading back home instead of on her ski trip.

She pulled over while driving to the ski weekend near Quebec City, trying to call Turcotte back several times without success.

By the next morning, the children were discovered dead.

The Crown also asked if she had had contact with Turcotte after the children were killed.

She said yes, May 17 that same year. She said she remembers that day because she thought she was going to kill herself, that she couldn't survive it all.

She called him to get their papers in order. Gaston broke down on the stand when she talked about the conversation. She said she told Turcotte that she loved her children more than herself; he said he loved them too.

There were times in court Monday when Turcotte, in the accused stand, cried.

The defence started its cross examination Monday, asking Gaston about the differences in their personalities.

Her testimony continues Tuesday.

Turcotte admitted last week to causing the deaths of Olivier, 5, and Anne-Sophie, 3.

The trial is entering its second week, but Quebec Superior Court Justice Andre Vincent told the jury there was a delay until Monday afternoon after a juror required medical treatment.

With files from Sidhartha Banerjee of The Canadian Press