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Kristen Johnston is used to getting grief about her appearance. After surgery for a burst ulcer in 2006 left her looking startlingly skinny even two years later, she fielded accusations that she was anorexic. "I couldn't believe the venom people had towards me," the 3rd Rock from the Sun actress recalls. "I really felt attacked." So when a producer expressed concern over her growing "belly" once she was on the mend, she took the criticism head-on. "I said, 'I don't have an eating disorder, and don't get me started,'" she recalls. "If people see an actress with a belly, then guess what: That's real. Deal with it."

Healthy, happy and more emboldened than ever to speak her mind, these days Johnston, 44, is an expert at dealing with what's real-though as she makes clear in her new memoir Guts, it was a long time coming. The book, out March 13, reveals a struggle the actress long kept secret: For years she was addicted to pain medication and alcohol, relying on up to two bottles of wine and a slew of Vicodin pills every day. "I hated myself, but I didn't think I could change," Johnston says. "I thought, 'This is who I am. I'm a mess.'" It wasn't until she spent two months in the hospital after her burst ulcer that she was forced to change. "I'm appreciative," she says, "that I'm still alive."

She says she fell into her addiction gradually. Though she experimented with opiates (first prescribed for migraines) while starring on 3rd Rock in the '90s, "I kept it at bay for a long time," she says. But when she started working on the stage in New York City in 2001, the theater lifestyle made her bad habits worse. "I'd go out every night after the show, then keep drinking by myself at home," she says. Soon she was using pills again. "Using pain pills when you don't need them is worrisome," she says, "but you block it out."

Stealing meds from her friends and "doctor shopping" to get prescriptions, "you are reduced to becoming a person you despise," she says. But Johnston remained highly effective at work, making it even harder to consider getting sober. "The hot messes crash and burn before too long, but if you are high-functioning, you justify everything: 'The doctor prescribed these. Everybody drinks after the play,'" Johnston says.

Friends noticed a change in the actress but found it difficult to confront her. "It was clear that something was off," Bravo exec Andy Cohen, a pal, tells PEOPLE. "She was always hilarious and the life of the party, but she had an issue with when to stop."

That decision was made for her when, after five years of abusing pills and alcohol, her ulcer ripped open, spilling the contents of her stomach into her body while she was in London for a theater production. In excruciating pain and barely able to eat, she lost 60 lbs. in two months and "really started to comprehend what I had done to myself," she says. But even then, she struggled to stay sober. One week after she left the hospital, a friend took her out for dinner and martinis. "I remember drinking that cocktail and everything in my soul was like, 'Stop it.' I didn't even want it," Johnston says. "That's when I realized it was a compulsion."

She headed to rehab at the Meadows in Arizona that same week, and there she confronted the loneliness she says was at the root of her problems. "I realized I was so unhappy just acting. I wasn't a vital person," says Johnston.

Today the actress, who lives in Manhattan's West Village with her rescue dog Pinkie, is working on TV again, starring in TV Land's The Exes, and keeps busy during her downtime. She teaches acting classes at NYU, is writing another book and founded Sober Learning and Motivation, an organization that is raising money to found a sober high school in New York City. "I have this terrible need to tell people that they are not alone [with their addictions] and there's something you can do," Johnston says. "It was one of the driving forces for me to write this book, and I think maybe it's keeping me sober."