In this week’s issue of PEOPLE, friends remember Cory Monteith, 31, who was found dead in a Vancouver hotel room. A troubled kid who beat the odds and made it big, the Glee star appeared to have overcome his struggles with drug abuse. But his sudden death from a lethal mix of heroin and alcohol leaves friends, family, and girlfriend Lea Michele reeling.

Monteith played a smooth-singing, sweet-talking jock on TV, but in real life he was fond of describing himself with one word: awkward. Even after the 6’3” star rocketed to stardom on the FOX smash Glee, he still “thought of himself that way,” says Maureen Webb, an early mentor who discovered Monteith when he was a 20-year-old self-described “wayward youth.” Dining with Webb and his manager Elena Kirschner at Vancouver eatery East of Main on July 11, Monteith – who had completed rehab for substance abuse two months earlier – “was in great spirits,” says Webb. Eschewing alcohol for rhubarb lemonade, “he looked so healthy,” she says. He talked about his recent four-day hike on Vancouver Island, and when the subject turned to his work with Project Limelight Society, a nonprofit arts program for kids, “his face lit up,” says Webb, who cofounded the group. Having overcome his own tough childhood, “he said, ‘The performing arts saved my life once. I want to give back.’”

But that was not to be. Just two days later Monteith was found dead in his room on the 21st floor of Vancouver’s Fairmont Pacific Rim Hotel of a deadly mix of heroin and alcohol, hours after returning alone from a night out with friends. As questions about his death and a possible relapse swirled, “This has rocked all of us,” costar Mike O’Malley, who played the stepfather of Monteith’s character, Finn, told PEOPLE. Adds a show source: “Everyone is stunned.”

It was a sudden end for a young man who had already faced a series of obstacles. A high school dropout who won his role on Glee at age 26 with a homemade audition tape and no vocal training, Monteith projected an air of low-key affability, but several insiders say that he and much of the young cast indulged in a lifestyle that comes with being single, rich, and famous. “They worked hard and they partied hard,” says another Glee source. At the same time Monteith quietly struggled with stardom. “Fame was hard for him,” says one friend. “It was something he knew he deserved but something he never quite knew how to handle.”

Monteith was respected for his unfailing professionalism on the set of Glee. “I never saw him come to work late, lose his temper, not know his line or argue with crew and castmates,” a show source says. Perhaps because he was so professional, his descent back into addiction was a longer slide than most people realized, says one source, adding that an intervention was staged prior to his most recent stint in rehab. “He was a very gracious guy, but his lifestyle was destructive.”

And yet in the past year he had found love and happiness with his costar Lea Michele, 26. The actress spent the days after his death receiving visitors at her Hollywood bungalow. Although the two had been traveling separately and hadn’t stepped out together since an event in L.A. on June 8, “they were very happy and very in love,” when Monteith died, says a source close to the actress. A friend who has spent time with her in recent days says she is shattered: “She needs to be left alone to grieve. Everyone is worried about her.”

More on Cory Monteith is featured in the 7/29/13 issue of PEOPLE, on newsstands now.

*Also In This Week’s ‘People’*
Exclusive Details: Why Leah Remini Left Scientology
Shannon Miller’s Baby Joy After Cancer: “In Motherhood, You Can’t Be Perfect All The Time’
Hollywood At Home: Stars Show Off Their A-List Spaces
Selena Gomez: “[Bradley Cooper] Blinded Me With His Eyes”