Ten years ago, Mischa Barton was the hottest starlet on television. Then came partying, drugs, and a terrifying trip to the psych ward. For the first time, the O.C. actress opens up in this week’s issue of PEOPLE about what really happened.

It was on July 15, 2009, when 23-year-old Barton was strapped to a gurney at Cedars-Sinai Media Center and threatened to kill herself. A few hours earlier, she had been confronted by her parents and agents at her Los Angeles home; she was under massive stress, working nonstop and partying even harder. Scared by her increasingly erratic behavior and worried she was in no condition to fly to New York for a scheduled appearance the next week, those close to her staged an intervention. Not long after the meeting, Barton blacked out and was rushed to the hospital. In the frenzy, Barton threatened suicide, which led to her being committed to the psych ward for four days. “It was a full-on breakdown,” Barton tells PEOPLE, speaking in detail for the first time about the events that short-circuited her career. “It was terrifying. Straight out of Girl, Interrupted. Story of my life.”

Today, Barton, 27, comes across as cautious and composed as she sips black coffee in a New York City office and looks back on how her life “spiraled out of control.” The London native and her family were unprepared for what would happen when, at 16, she landed the role of California dream girl Marissa Cooper on The O.C., a soapy teen drama that captivated pop culture. “Almost overnight it was like this switch had been turned on,” she says of the blazing spotlight cast on the show’s young stars. “It was like this fascination switch on all of us, aimed especially at me.” Suddenly, no temptation was beyond reach. “We thought, ‘Work hard, play hard,’” she says of partying with Lindsay Lohan and Nicole Richie, jetting around the world and raking in millions for endorsements. “It was a train I could not get off of. When you’re young, you can do it, but after a while, it’s going to come crashing down on you.”

Barton admits she and her mother (who declined to comment for this story) clashed at times, but says both of her U.K.-bred parents suffered from culture shock in Hollywood. “They were thrust into this situation that was completely foreign to them,” Barton says. “Nothing could prepare them to have their children jump into the overtly sexualized and crazy world of L.A.”

Despite her threat at the hospital back in July 2009, “I was never suicidal,” Barton now insists. “I was just overworked and depressed. But one slip of the tongue in a heightened moment and you find yourself in that situation.” Her time in the psych ward was “an eye-opener,” she says. “I was deeply hurt at first, and then I accepted this was time I needed to be away from work, my family, and all the pressure. I had been through the wringer.”

She retreated to Paris for more than a year, and then London. “I needed to be on my own and get healthy,” Barton says. But that led to new criticism when she noticeably gained weight. “When you’re 16, you’re not what you are at 25,” she says matter-of-factly. A size 0 in her O.C. days, “I was really young and just had not filled out at all. Not everybody stays the same body type. It was always, ‘She’s too skinny, she must be sick.’ Then it was, ‘She’s too big.’ I was never the right weight.” Today, Barton says, “I just try to be balanced.” And she notes that she’s happily single. “I’ve been in relationship after relationship,” she says. “I love men, but it’s kind of nice to be by yourself and take that leap of faith.” Such self-reliance has been hard-earned. “I needed to take this time for me,” she says. “I’ve learned a lot. I’m stronger now, and I’m excited for what’s ahead.”

More on Mischa Barton appears in the 10/28/13 issue of PEOPLE; on newsstands everywhere.

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