UFC legend Georges St-Pierre -- one of the greatest of all time -- says he actually HATED fighting during his 15 year MMA career.

... but he stuck around so long because he was addicted to the feeling he got after winning a match!

The 39-year-old superstar opened up about the situation recently on "The Complex Sports Podcast" ... and didn't mince his words.

"I don’t like the fighting. I hate it. It’s unbearable," GSP said ... "The feeling of stress. Not knowing if you will be humiliated or you might get hurt. It’s so hard."

"But when you win a fight, it’s really worth it. The bigger the risk, the bigger the reward. So that’s why I did it. Not because I love to fight. I love to win."

And one thing GSP did a lot during his career was win -- finishing his MMA career with a 26-2 record ... with victories over legends like BJ Penn, Michael Bisping, Nick Diaz, Matt Hughes and more.

GSP formally retired in 2019 -- 2 years after his last fight -- and has repeatedly said a comeback is most likely off the table.

Now, he's focusing on acting -- reprising his role as the French mercenary Georges Batroc in the upcoming Marvel series, "The Falcon and the Winter Soldier."

Georges says he uses some of his skills from fighting to help him as an actor.

"Like in fighting, acting requires a lot of repetition, a lot of preparation. In fighting, you do a lot of those as well and very often when you fight, you find out your opponent is never as good as you think he is and he’s never as bad as you think he is as well. It’s always different."

The main difference ... GSP acknowledges the stakes are much higher in fighting.

“In fighting, you only get one take. If you zig when you should zag --  boom! It can cost you not only a loss but it can cost you obviously cerebral damage. Damage that can cause your death. It’s very dangerous."