Cory Monteith first overdosed when he was 15 years old.

The 'Glee' actor - who died from ''mixed drug toxicity'', including heroin and alcohol last July aged 31 - had struggled with addiction for a long time, and his mother Ann McGregor thinks much of his inner turmoil came from his lack of relationship with his father, her ex-husband Joe Monteith.

Ann said: ''[When] Cory was 15, he did a code blue, and I think that was the turning point.

''He had a lot of emotional things he was trying to figure out -- lot of it was he really wanted a relationship with his father. I think when a child gets invalidated they keep reaching even harder. They want to find out why.''

Ann - who is still in touch with Cory's former girlfriend Lea Michele - doesn't think anyone was able to help her son, who underwent a stint in rehab in March 2013, at the point of his death.

She said in an emotional interview on 'Good Morning America': ''I don't think we have power to change the choices they're making.

''I think with the kind of connection Cory and I had, if we couldn't prevent that situation, I don't have the answer.''

Ann, 63, is still struggling with her grief over the loss of her son and admitted it is only now she is able to look at pictures of him again.

She said: ''The loss is horrendous. Until three days ago I couldn't look at a picture of Cory... So there's been progress.''