Rejection 'ultimate insult' to narcissist Donnison
To former City worker Fiona Donnison, being rejected by her partner Paul would have been the "ultimate insult".
Killing her children, three-year-old Harry and two-year-old Elise, was done to get back at him, the jury was told.
The prosecution said she was not "simply mad" but a narcissist with an over-developed sense of self-importance who planned the killings.
Psychologist Dr David Nias said she would have been unable to understand why Mr Donnison lost interest in her.
"It would have been a continual crisis for her," he said.
'Get own way'
"For someone with the narcissist personality disorder rejection is the ultimate insult.
"They just cannot understand why someone would want to reject them because they see themselves as special and they expect to always get their own way."
She was found guilty of murdering her two young children on Tuesday following a trial at Lewes Crown Court.
The couple split up in September 2009 when Ms Donnison moved out of their detached home in Heathfield, East Sussex with the children.
They tried to get back together but decided to end their relationship for good on 15 January 2010.
The children's bodies were found in holdalls in their mother's car boot on 27 January.
Ms Donnison had believed Mr Donnison was having an affair with an old school friend, Alison Shimmens, during the attempted reconciliation.
He admitted in court he had received "flirty" emails from Ms Shimmens but maintained the relationship was platonic until the break-up with Ms Donnison.
Mr Donnison told the jury Ms Donnison was a good mother who could be "very nice, very pleasant" and the home they also shared with her two teenage sons from a previous marriage was happy.
But he said her mood could change quickly and she could be provocative, antagonistic and confrontational.
"Almost by definition, if someone who is a good mother then kills their children it must be because they have temporary insanity," said chartered clinical psychologist Dr Nias.
"Killing is of course very much a spur of the moment thing so it's incredibly difficult to know exactly what was going through the person's mind at the time they commit the act."
The jury at the month-long Lewes Crown Court trial heard conflicting evidence from medical professionals about Ms Donnison's state of mind.
She had faced a number of stressful events in the months leading up to the children's deaths, including being made redundant and moving house, and claimed she was depressed and not in her right mind.
But the prosecution maintained she planned the killings and was not "simply mad".
"When people are under a lot of stress it does just need a very small trigger to push them over the edge to then commit an atrocity," said Dr Nias.
"It's almost as if in a split second they become insane and then they can be back to normal when the shock of what they've done hits them.
"What usually happens is that a combination of factors come together in a particular situation.
"Spousal revenge is one of the terms that's used because often killing the children is apparently because of a dispute between the parents.
"Fortunately killing children is an extremely rare offence.
"The mother bitterly regrets what's happened. Of course it is something that they have to live with for the rest of their lives."