A source tells Us Weekly that Cory Monteith was "not a typical addict" and managed to hide his lifelong battle with drug abuse. Credit: Jeff Vespa/Getty Images

Behind Cory Monteith's lovable jock character on Glee was a man locked in a lifelong battle with drug addiction. The 31-year-old British Columbia, Canada native's secret life -- months after a rehab stay -- was exposed when he was found dead in his Vancouver, British Columbia hotel on July 13 after a taking a lethal mixture of heroin and alcohol.

"He was not a typical addict where it made him evil," a confidant tells Us Weekly. "He was the nicest, sweetest guy."

Monteith had been a bright child, reading at the fourth-grade level by age 5, but with his parents' divorce at 7, he began acting out. At 13, he was ditching class to smoke pot and drink, eventually attending 12 different schools before age 16. 

"I was out of control," he told Parade in 2011, admitting he used "anything and everything, as much as possible." He entered rehab at 19. "I did the stint but then went back to doing exactly what I left off doing." 

He got his big break when, on a whim, he sent an audition tape for Glee, earnestly singing an REO Speedwagon power ballad. But even after getting his big break on Glee, Monteith's dangerous addiction haunted him. 

"He really struggled. Essentially, he was a street kid," a source tells Us. "But he looked so angelic and he was playing this clean-cut guy."

"This was a guy who spent more time in back alleys in Vancouver doing what street kids do than walking red carpets," the insider explains. "That was his life. He was still coming to terms with that."

Monteith is survived by his parents Ann and Joe, brother Shaun and costar/girlfriend Lea Michele. "She didn't know he'd started using again," a source tells Us. "She's not doing great. She's totally devastated."