Jim Parsons Says He Was 'Miserable' During 'Big Bang Theory' Stardom

Jim Parsons says his climb to the heights of sitcom fame didn't leave him in the best mental space ... admitting he was "miserable" during some of the biggest moments of his life.

The actor -- who is currently starring in the hit Broadway show "Titaníque" -- opened up about his trial and tribulations while working on "The Big Bang Theory" in an interview on Jon Dean's "All Out" podcast ... explaining he overworked himself because he felt it's what the job demanded.

He explained, "I felt that there was so many plates I was supposed to be keeping in the air and that the success and the good things of life that were happening were only due to this overworking ... discipline and whatever. Maybe to a degree that was true. I don’t know. I can’t say because that’s how I was.”

While the success of 'TBBT' did wonders for Parsons' career -- and reportedly earned him millions of dollars -- he now says he wouldn't do a project again if it made him feel that way ... not even for a massive paycheck.