July 19, 2018 08:00 AM

In the five years since former Glee star Cory Monteith died of a heroin overdose at age 31, his mother Ann McGregor has had time to plenty of time to reflect on her enormous loss.

“A part of me died when Cory did,” McGregor, 67, tells PEOPLE. “My world stopped and I didn’t think I’d ever get up from it.”

Monteith was just 13 years old when he became mixed up with an older crowd and began experimenting with drugs and alcohol.

“He was a very vulnerable young man all the way through his life,” says McGregor. “I raised him to be honest and open and I raised him with love. But I did not teach him about the other side of the world. There’s a bad side out there.”

Courtesy Ann McGregor

By age 15, McGregor checked her son into rehab, but after a month-long stay, he began using again and was back in rehab at 19.

“I realized that rehab is not the answer,” says McGregor. “You have to get these children before they get into that stuff. I never had the power to stop him.”

Still, Monteith was sober for a time, and after moving to Los Angeles, found success as an actor in small parts before his star-making role on Glee.

Courtesy Ann Mcgregor

But Monteith never felt comfortable with the fame and attention. “He called that world plastic,” says McGregor. “It was too superficial for him. He was just too grounded and his heart was too intact. He couldn’t become hard enough.”

McGregor says Monteith began using drugs again as a way to escape the pressures of Hollywood. “He was stressed because he wanted to get out of that world but he couldn’t because he had two more years left on his contract. Drugs were his way of checking out.”

  • For more on Cory Monteith, pick up this week’s issue of PEOPLE, on stands Friday

Monteith checked into rehab for the third time in April 2013, but on July 13, 2013, he was found dead in a Vancouver hotel room with traces of heroin, morphine and codeine in his body.

“He didn’t want to die,” says McGregor, who visits her son’s ashes, which are scattered on a peaceful beach in British Columbia, on a regular basis. “There’s no doubt about that. He was looking forward to his future. And he had so much to live for.”

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