"But at the time, I thought, 'Okay, my arms look awful so I should cover them up, but my legs look pretty okay now," says the actress, 37. "I didn't think I was crazy to only eat 300 calories a day. That made perfect sense to me."
In a new memoir, Unbearable Lightness: A Story of Loss and Gain, de Rossi comes clean about the years she spent battling anorexia and bulimia – after landing her breakthrough role on TV's Ally McBeal in 1998 – and the pressure she felt to hide the fact that she was gay.
"My struggle with anorexia, coming to terms with my sexuality, I realized I had struggled with self-acceptance my whole life," she says. "To be able to get to a point where you're just happy with who you are, I think that's when you know what real peace feels like."
Now healthy and happily living in Los Angeles with Ellen DeGeneres, whom she married in 2008, de Rossi never steps on a scale and eats three balanced meals, plus snacks, every day.
"Look, I don't think I'm perfect. I still don't like my thighs," she says. "But I'm not going to do anything to compromise my health or my sanity to change them. I don't want to have secrets anymore. I don't want to have a darkness that I feel that I should be ashamed of. And going forward now, without having anything to hide, the future looks pretty bright."
For more of de Rossi's interview and an exclusive excerpt of her book, pick up this week's issue of PEOPLE, on newsstands Friday