11/26/2007 at 06:00 PM EST
Success in her various professional guises, and her romance with best friend Pete Wentz, have Ashlee Simpson
sounding positively giddy about life these days – and more resilient than she used to be.
Chief among her confidence-boosting achievements of late – her 2006 stint as Roxie Hart in the London production of the musical Chicago
, Simpson tells CosmoGIRL
in its December/January issue.
"There was a time when what people said about me could really get to me. But working on Chicago
helped me come into my own," says Simpson, 23. "[Theater acting] had been a dream of mine ever since I was little, and being able to fulfill it ... was amazing. It taught me to be more comfortable with myself."
That self-possession, she says, helped her deal with fame better, too.
"I've gotten to the point where being credible with myself is more important to me than what anyone else says," she explains. "People make up stories about you, but they don't really know you. ... I'm not fazed by that anymore."
Simpson also credits boyfriend Wentz, 28, with providing encouragement in her private life that has boosted her professional work – in particular, her third album, due out next year.
"He's a great sounding board and very supportive," Simpson says. "Pete and I compliment each other. We are best friends, and we encourage each other. I think that is so important in any relationship – it builds a solid foundation."