As the it-girl actress and Girls creator reveals in her new collection of personal essays, Not That Kind of Girl, she battled severe anxiety and obsessive compulsive disorder as a child – and, she tells PEOPLE, she still wakes up every day with a certain sense of dread.
"It's like, 'Oh, you again?' " says Dunham, 28. "I really love my life, but I've kind of worn anxiety grooves into my brain. It just doesn't always sound good in there."
Unhappiness about her body was often part of the mix, despite the ease with which she undresses for the camera.
"When I got out of college I thought, 'What am I gonna do? No one's gonna hire me, I'm a fat girl,' " she says.
She devotes a chapter in her new book to the obsessive food journal she kept in an attempt to lose weight as she started shooting for her HBO show's first season. (She also talks about those extreme dieting days in one of twelve book-trailer videos she's posted on YouTube.)
"My food intake was a hard thing to share publicly," she says. "A lot of my life and work is sort of about not succumbing to [those pressures], so it's a little painful to go, 'Oh but look, there was a time where this dominated every moment of every day.' "
Her eating was so restricted that she ended up "in the hospital with crazy stomach pains. It was basically revealed that I'd been just drinking laxative tea and coffee and smoking cigarettes and then eating weird foods at weird hours. I really messed myself up," she says.
All that changed, Dunham says, thanks to her "incredibly supportive boyfriend," Fun.'s Jack Antonoff, as well as to "getting busy and realizing that food was fuel" and simply getting older.
"I really feel good with my size now," she says. "I know when I say that people are like, 'mm hmm', but I just do! It used to be when I went into a room with all thin women I felt like, what's wrong with me? Now I just feel special."
Not That Kind of Girl hits stores Sept. 30.
For more of Dunham's interview, pick up this week's issue of PEOPLE, on newsstands Friday