The actress and singer explained that Monteith - who plays her character Rachel Berry's on-off boyfriend Finn Hudson on the Fox comedy - knows her better than anyone else.
"I don't even remember a time when he wasn't my boyfriend. No-one knows me better than Cory," Michele told Marie Claire.
"No-one knows what it's been like to go through this more than he does. Feeling like you have that net underneath you allows you to jump higher and go farther. He makes me feel like I can do anything."
Michele also revealed that she has now got over people labelling her a "diva", but still finds it "frustrating" at times.
"I can assure you that [the diva rumours] aren't real," she said. "It's super-frustrating sometimes, and I used to fight back, but now I feel like, 'You can think what you want to think about me - that's okay'."
She added: "I have the most amazing fans, friends, and family members who have stood by me, and I know that my cast and crew really enjoy working with me. That's all that matters."
Lea Michele recently described her breasts as her "prizewinners", saying she now feels comfortable showing off her cleavage.