Lea Michele -- Controlling Cory Monteith in Life and Death

Cory Monteith had a very strong guardian angel by his side ... his girlfriend Lea Michele, who essentially controlled Cory in life and even death.

Sources connected with both Cory and Lea tell TMZ ... Lea swooped in when Cory died and immediately took over planning his memorial and other matters connected with his death. We're told Cory's mom is not involved at all in the planning -- it's all Lea.

Everyone agrees ... Lea always had Cory's back and tried protecting him as much as she could, especially when it came to his substance abuse. We're told Cory -- who was sweet but naive -- got banged around in Hollywood by what one relative called "a zoo of piranhas" and when Lea became his girlfriend she fiercely protected him.

As for Cory's drug and alcohol problems ... Lea laid down the law in the United States and did everything she could to keep him from relapsing. When he did relapse earlier this year, she was the one who took the lead and engineered the plan to get Cory in rehab.

We're told Lea's problem was that when Cory went to Canada ... all bets were off.