The actor was found dead at the age of 31 in a Vancouver, Canada, hotel room. According to authorities, he died of a "mixed drug toxicity" of heroin and alcohol.
"I got a call from Lea [Michele, Monteith's girlfriend of over a year] and she was screaming on the phone," McGregor, 67, told PEOPLE. "She was yelling, 'Is it true, is it true about Cory?' and I said, 'What about Cory?' I had heard nothing. And then police knocked on my front door."
After she heard the news, McGregor recalls entering in a state of numbness and shock.
"I just shut off all emotions, and I was numb," she told the outlet.
As days and weeks passed, "I knew it was real, but there were times when I could lie to myself and say, 'He's in L.A., he'll call me soon.' I'm still always hanging on edge. When you lose someone who is such a big part of you, you lose all purpose for living."
Monteith had been open about his struggles with drugs and addiction and had been in rehab three times. Just before his death, however, he had been making plans for the future, renting an apartment in Vancouver where he planned to move once his contract with Glee was up and even talking to his mom about starting a family.
"Cory wanted a wife and kids," McGregor said. "He would have made a beautiful father. He had accomplished so much. He was ready to step out of Hollywood and really live. He had so much to look forward to."
Today, McGregor focuses on keeping Monteith's passion projects outside of Hollywood alive, including his work with Project Limelight, a non-profit, free performing arts program for children, and St. James Music Academy, which provides classical music education for at-risk children. McGregor also works with Amber Academy, a non-profit in her community that empowers youth through fine arts.
"Cory didn't want to die," McGregor said. "There is no doubt about that. But he's carried in people's hearts forever."
Also in her conversation with PEOPLE, McGregor revealed her son had checked into a month-long rehab stint in April 2013, followed by "massive dental work between May and July, which could have affected his sobriety.1comments
"He had little teeth and they were all capped," she said. "He had a lot of medication in his system, which was not good for his body coming out of rehab."
"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]," she added.