Drouin, 26, told TSN/RDS in an interview Monday that he's been dealing with the issue for many years, and his breaking point came during a road trip last season.
The Canadiens played two games in Edmonton, and Drouin appeared in both. He took part in the pregame skate before their game at the Calgary Flames on April 23, but left the lineup for what the team said was a "non-COVID related illness." He missed all three games the Canadiens played in Calgary on that road trip and would not appear for Montreal again for the rest of the season.
Drouin hadn't slept for three nights before the Flames game.
"That's where it hit a wall for me. It was time to step away from the game. Literally take a step back from everything and enjoy life," said Drouin, making his first public comments about his leave of absence. "It was hard for me to do at that time -- obviously, the playoffs were coming around."
"It wasn't an easy thing to do. But I'm proud of what I've done and I'm happy I did it," he said. "It's hard to step away and watch all the guys. But at that time, I made my decision and stuck with it."
Drouin will take part in Canadiens training camp, which officially begins on Wednesday.
"It's just good to be back to my training and my skating. I took a lot of time off to think about stuff. Just happy with how my summer went," he said.
Drouin has 232 points in 393 NHL games with the Lightning and Canadiens. He had 23 points in 44 games with Montreal before leaving the team.