Scottish singer Shirley Manson attributes her trust issues to her traumatic first sexual experience.

The Garbage star previously revealed she engaged in an underage sexual relationship with an unnamed Edinburgh music teacher when she was 15-years-old, telling the Sydney Morning Herald she would have sex with him in the back of his van during a 2013 interview.

But now the 49-year-old has opened up about a scary experience she had with a boy when she was just 13.

"He grabbed a knife and told me he was going to stick it up my vagina," she tells "It frightened the s**t out of me."

The rock star admits the incident triggered a series of issues for her, adding, "I think that's where it all began to unravel.

"I just had this realisation: 'Why am I distrustful all the time?' It's probably because of that. I spent so long ­disgusted about myself. I would sob in the ­bathroom, because I thought I was failing."

Shirley claims school bullies and her tough mum also left her battling self-esteem issues throughout her teenage years.

"My mom was always the hardest on me because I looked the most like her," she says. "She would tell my two sisters they were beautiful, but tell me I had a great personality. I just thought I was an ugly b**tard."

Despite her struggles, Shirley has a brighter outlook on life these days although she still wrestles dark feelings and emotions from time to time.

"I'll feel really grateful for the success I've enjoyed, but then, at some point in the month, a dark Shirley returns - and her voice gets really loud," she adds. "I'll feel like a f­**king idiot because I don't have goals. I have self-doubt and ­frustrations and disappointments. I think it's part of the human condition. Even someone like Beyonce doesn't f**king believe in herself 24/7!"