"I needed to make a change because I was tired of the way I was living."
That's 31-year-old former heavyweight champion Andy Ruiz opening up about depression that literally brought him to his knees ... after losing his belts to Anthony Joshua.
TMZ Sports talked to Ruiz days before he makes his return to the squared circle on May 1 ... where "The Destroyer" will take on Chris Arreola.
When Ruiz starts letting his hands fly ... he'll look a little bit different than the last time we saw him in the ring with Joshua -- 'cause the 6'2" boxing star has lost nearly 60 pounds!