04/11/2013 AT 4:00 PM ET
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.”