Updated October 28, 2013 12:00 PM

Strapped to a gurney at Cedars-Sinai Medical Center, Mischa Barton threatened to kill herself. It was July 15, 2009, and the 23-year-old – who had rocketed to fame as the star of the FOX series The O.C. – was frightened and frantic. A few hours earlier, Barton had been confronted by her parents and agents at her home in Los Angeles; 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 closest to her staged an intervention. The young actress felt the walls closing in, and she had taken a powerful sedative to combat anxiety. Not long after the meeting, Barton blacked out and was rushed to the hospital. There, she became more distraught and was restrained by staff after she fought them and tried to leave. In the frenzy, Barton threatened suicide. That led to her being committed to the psych ward for four days under California’s 5150 hold (the provision used to force Britney Spears and Amanda Bynes into treatment). “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 became the target of relentless tabloid gossip criticizing everything from her excessive partying to the size of her thighs: Blogger Perez Hilton ridiculed her as “Mushy Fartone”; another website called her “a bloated shell of herself.” Dressed in skinny jeans that accentuate her statuesque height (she’s 5’9″) and now-healthy curves, Barton admits the mockery wounded her deeply. “When you’re under a microscope, you learn to switch it off,” she says. “I don’t think you realize how hurtful it is until much later.” That’s part of why she chose to speak out: “You can only take so much before it affects you, people giving you sideways looks on the street. It gets to the point, ‘Okay, I’m going to put my foot down and say something.'”

As she ventures back into the spotlight with new movie projects and a TV pilot, Barton isn’t bitter. “She has this carefreeness and optimism that you get from going through all that stuff and coming out the other side,” says Ryan Eggold, her costar in the supernatural thriller I Will Follow You into the Dark. She’s also trying to go easier on herself this time around. When she was a teen, “everybody was depending on me,” she says. “I asked to get out of jobs all the time, and the response was, ‘No, you have to,'” says Barton. “There’s an attitude that you can’t say no.”

Barton’s mom, Nuala, served as her manager and earned a reputation as a tough stage mother. “Mischa was the bread and butter for her entire family,” says a former associate who worked closely with her. “But her mom was running the show.” 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,” she says. “Nothing could prepare them to have their children jump into the overtly sexualized and crazy world of L.A.”

Barton moved from London to New York City with Nuala, her financier father, Paul, and sisters Zoë and Hania when she was 5 years old and landed her first theater gig three years later. Her ethereal looks and precocious talent soon scored her a breakout role in 1999’s box office smash The Sixth Sense. But it wasn’t until The O.C. took off in 2003 that things began to hit hyperdrive. “I was living a jet-set lifestyle,” she says of those years. “There were a lot of enablers around, people to fly you around and to make it all possible.” (Her younger sister Hania, 25, struggled too and reportedly entered rehab in 2007 for painkiller abuse.)

Barton’s love life was similarly drama-filled, including flings with wealthy bad boy Brandon Davis and rocker Cisco Adler. “We were madly in love,” she says of Adler. But even as her romances generated attention, her career began to stall. In 2006 her O.C. character was killed off in a car crash, a move the show’s creator Josh Schwartz recently called a “creative decision” that “had nothing to do with [Mischa].” Then in 2007 she pleaded no contest to a DUI and driving without a license, resulting in court-ordered rehab and supervision from a sober coach. Today, Barton still hedges when discussing her drug and alcohol use, reluctant to see herself as having been an addict. “I think I was just really stressed out,” she clarifies. “I don’t think any of it would have happened if I hadn’t been under such enormous pressure.”

In 2009, while playing a model on the CW series The Beautiful Life, she hit bottom, and her parents decided to confront her. Barton says she was reeling from botched wisdom tooth surgery and was prescribed Xanax for anxiety. She had never taken it before and blacked out. Despite her threat at the hospital, “I was never suicidal,” she insists now. “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,” she 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, she says, “I just try to be balanced.” That means “no food don’ts” and regular classes at SoulCycle. She also has an occasional drink: “I have a glass of wine with dinner. It’s just moderation, really.” She splits her time between her apartment in London (where she has a fashion boutique that carries her handbag line and favorite bohemian dresses) and her home in the Hollywood Hills with her three dogs Charlie, Ziggy and Harvey. 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.”