Cory Monteith's Mom Says He Was on Pain Meds for Dental Work Weeks Before His Tragic Overdose

It’s been five years since former Glee star Cory Monteith died of a heroin overdose in 2013, but for his mother Ann McGregor, it’s like no time at all has passed.

“I still can’t pick up the pieces,” McGregor, 67, tells PEOPLE of losing her younger son when he was just 31. (Her son Shaun is 38.) “My world totally stopped. And I’m a different person than I was before.”

Monteith, who was raised by single mom McGregor in Victoria, British Columbia, was very close with his mother growing up.

“We used to go everywhere together,” recalls McGregor in this week’s issue of PEOPLE, on stands Friday. “We had a favorite restaurant where we’d share fish and chips. Even at 3 years old, he would stand at the counter and pay the bill. He could talk to anyone. He just had a way about him, and all I could do was enjoy it.”

As a child, the “advanced” Monteith (he learned to read at just 3 years old) skipped two grades and was hanging out with an older crowd by his early teens.

“There was this disconnect,” says McGregor. “Because he was pushed so far ahead and always associated with older children, but he was still a boy.”

Monteith began drinking and smoking marijuana at 13. By the time he was 19, he was in rehab for the second time.

“He tried to keep everything from me, because he loved me and wanted to protect me,” says McGregor, who took Monteith to rehab at 15 and then again at 19. “He was just always so curious. And the darker world just drew him in.”

Eventually, Monteith got sober and moved to Los Angeles, where he got bit parts on television before landing a starring role on Glee.

But as busy as he was, Monteith always made time for mom.

“He never missed my birthday,” says McGregor, who notes that Monteith used to show up at her door as a surprise and would often send her plane tickets to come visit in L.A.

But eventually, Monteith fell back into old habits. “He wasn’t ready for the Hollywood world,” explains McGregor, who says her son admitted he was using drugs again in December 2012, seven months before his death. “Drugs were his way of checking out.”

Monteith checked into a month-long rehab in April 2013, but between May and July, he had “massive” dental work that McGregor worries could have affected his sobriety.

“He had little teeth and they were all capped,” says McGregor. “He had a lot of medication in his system, which was not good for his body coming out of rehab.” (When Monteith’s body was discovered in a Vancouver hotel room on July 13, 2013, there were traces of morphine, codeine and heroin in his system.)

Continues McGregor: “He didn’t have enough drugs in his system to kill him, but for some reason it did because of his intolerance [built up by periods of intermittent sobriety].”

Now, McGregor is focused on remembering her son and continuing his legacy.

Monteith worked with Project Limelight, a non-profit, free performing arts program for youths, as well as St. James Music Academy, which provides classical music education for at risk children.

“Cory believed in prevention, rather than trying to fix people,” says McGregor, who herself is working with Amber Academy, a non-profit in her community that empowers youth through fine arts. “He wanted to give children opportunities to shine and feel good about themselves so they wouldn’t turn to drugs.”

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

And not a day goes by, that McGregor doesn’t think about her own son — and the memories he left behind.

“Cory was such a loving and genuine person,” says McGregor. “And the best part of his life was still ahead.”