Police were called to Miss Mayer's house in Watton-at-Stone in January after she failed to pick up her children from school, the Daily Mail reported.
The inquest heard that she was found dead in her pyjamas. A postmortem examination found 10 knife wounds to her head and neck and a number of wounds from a blunt object.
After the stabbing, Barrett, who had two children from a previous relationship, washed himself and dressed.
The civil engineer cleaned and put away the knife before hanging himself, coroner Edward Thomas was told.
The couple moved in together in December 2010. The Daily Mail reported that Barrett killed his partner a month later after they argued because he did not want the children around over Christmas.
Detective Constable Alex Warwick, from Hertfordshire Police, said Barrett was jealous of Miss Mayer's sociable life and even her children.
She told the court Barrett's father had been jailed for murdering his mother in the 1980s in New Zealand.
"On Rosie's suggestion he was seeing a counsellor and had four sessions about how he felt in relationships and what had happened to his parents.
"In his counselling sessions it came out that he felt Rosie gave attention to her children and her ex partner over him. It was petulant attitude."
Coroner Thomas recorded a verdict of unlawful killing and suicide.
He added: "I hope people will be supported perhaps in a way that Mark wasn't when it happened to him when his father killed his mother.
"He had no one then. I think Rosie saw that and tried to help with counselling, she cared about him. They hadn't rushed into the relationship.
"She had a busy life and I feel that must have been difficult for Mark. She was vivacious, attractive and everyone loved her and he found that hard."
