After getting together with Jay Nava, Natasha Wake became "quickly infatuated", Shelley Powell told Winchester Crown Court.
The prosecution alleges Mr Nava fatally stabbed her while her children slept upstairs at her Bournemouth home.
Ex-soldier Mr Nava, of Royal Citadel, Plymouth, denies Miss Wake's murder.
Mrs Powell said despite the early infatuation the relationship became off-on, and her 26-year-old daughter finally ended it when she discovered Mr Nava had been unfaithful while she was pregnant with his child.
In a statement, Mrs Powell said her daughter once admitted she was "frightened" when she realised Mr Nava had problems with alcohol.
During her daughter's relationship, which started in 2013, Miss Wake became "isolated" from her friends because Mr Nava did not like socialising with them.
"She spent less time wanting to go out, she couldn't take him to anything where he might want to drink, it could be like a switch was flicked and he became a different person," Mrs Powell said.
After the split, the court heard Miss Wake remained friends with Mr Nava and he visited their child regularly, with him sometimes staying at her home, but Mrs Powell said her daughter was trying to "move on with her life".
Jurors heard Mr Nava, who was a serving soldier at the time of the killing, had been investigated by police in May 2016, after a woman alleged he sexually assaulted her with another man.
Miss Wake found out about the investigation on 1 October last year and day later her body was found wrapped in a duvet in a cupboard under the stairs at her home.
The trial continues.
A woman had a "volatile relationship" with her ex-boyfriend who is accused of stabbing her 11 times, her mother has told a court.