From PEOPLE Magazine Click to enlarge
Cory Monteith looked as if he didn't have a care in the world. It was March 16, and he was in Vancouver for a pal's wedding. On his arm was his castmate and girlfriend Lea Michele, and as usual the Canadian actor, who overcame drug problems in his teens to become a wholesome star of Glee, had a ready smile for all. "He had his arm around Lea, and they were laid-back, friendly, genuine," says Shawn Miller, who officiated at the ceremony. "They seemed chilled out and happy."

Few knew then that Monteith, 30, was struggling with past demons. But within weeks he checked into rehab for substance abuse at the urging of Michele and others, according to an insider. Sober for years after a rehab stint in Canada, sources tell PEOPLE he had recently fallen back hard into old habits. "It was more than booze," says the insider. "This latest slip came as a surprise. But people seem pretty certain he'll get better. He's been down this road before."

In a town where the addiction-related flameouts of promising young stars are depressingly common, Monteith's woes have been met with sadness. On the Glee set, "his work ethic is strong, and he always delivers," says the insider. "He was never known as one of the troublemakers." But he was also surprisingly candid about his issues before finding fame as Finn on the Fox hit. "You see this young All-American quarterback-looking dude on the show," he told a Canadian interviewer in 2011. "If I can ... shed light on the way out of a difficult situation that I know that many kids are experiencing like I did when I was a teenager, that's huge."

Growing up in Victoria, B.C., the youngest son of Ann, an interior designer, and Joe, a military serviceman who is now retired, Monteith "had a rough teenage period," says Ann McHugh, his boss when he worked as a night taxi driver in Victoria. At age 19 he went to rehab for alcohol, marijuana and hard drug use. "[I took] anything and everything, as much as possible," Monteith later told Parade. "I had a serious problem. I was out of control. I am lucky to be alive."

For years Monteith remained committed to his sobriety. As an actor starting out in Canada, "he wouldn't drink anything at all," says a hometown source. "Not even Coke, just water." Once he made it in Hollywood, "his choice to drink again happened gradually," says a friend. "He started out having one drink on occasion. Eventually, I would see him and he was wasted. He acted differently when he was drunk."

Still, Monteith largely stayed out of the Hollywood club scene, instead hosting weekly parties with his roommates for his closest friends and cast members. Though sources say the parties often lasted till dawn, "they just hired deejays and people danced, but I never saw anything crazy," says a source. Once he began dating Michele a year ago, the actress, known to be extremely driven and focused, proved to be a positive influence. "They would drink," says the source. "But they drank in a way that seemed responsible."

Because of Monteith's reputation for professionalism - "He's not a huge diva or jerk," says the insider, "he was always a team player"—many around him missed signs of trouble. "For a person who has a lot of practice doing drugs, the pattern is well-scripted in the memory," says Steve Sussman, a professor of preventative medicine and psychology at USC's Keck School of Medicine, who is not treating Monteith. "It's easy to try and forget it, but it's always there."

In recent months the actor, whose role on the show has been scaled back this season, appeared to be losing weight. "It looked like a healthy weight loss, like he was getting into shape," the insider says. A source adds, "It raised some eyebrows. People were saying, 'I hope everything is okay.'"

With his rehab stint now underway, friends say that Michele has been his rock. "Lea was good for him," says a member of Monteith's circle. "She's really calmed him down." And like all of the actor's friends, she is hoping for the best. "His going to rehab is a positive thing," says one of Monteith's hometown friends. "He is doing the right thing." Adds another source: "It's surprising when someone you know goes into treatment if they're not openly doing drugs, but relapses happen. We just didn't think it would happen to Cory."