The Canadian actor admits he started drinking and smoking marijuana at the age of 13 before dropping out of school at 16 and starting to use harder drugs.
He told Parade magazine he was doing "anything and everything, as much as possible - I had a serious problem. I burned a lot of bridges."
His family felt his abuse could lead to his death by the time he was 19, prompting them to intervene.
He added: "That's when I first went to rehab. I did the stint but then went back to doing exactly what I left off doing."
Cory's life continued on a downward spiral until he was caught having stolen a "significant amount of money" from someone in his own family, and given an ultimatum to get clean or be handed over to the police.
He explained: "I knew I was going to get caught, but I was so desperate I didn't care. It was a cry for help. I was confronted and I said, 'Yeah, it was me.' It was the first truthful thing that had come out of my mouth in years."
Cory, 29, managed to turn his life around by moving in with a friend and getting a job as a roofer, which is when he found acting and started on the path which would lead him to appear in 'Glee'.
He warns others his Example is not to be followed, however, saying: "I don't want kids to think it's okay to drop out of school and get high, and they'll be famous actors, too.
"But for those people who might give up: Get real about what you want and go after it. If I can, anyone can."