War Machine's story made national headlines -- he was a super talented UFC fighter who lost it all in 2014 when he viciously attacked his then-girlfriend, Christy Mack, when he caught her with another man.

Mack's injuries were horrific -- 18 broken bones, broken eye socket, broken nose and a lacerated liver. The other victim, Corey Thomas, was choked out and bitten before escaping.

War Machine was eventually convicted on 29 felony counts and sentenced to life in prison with the possibility of parole after 36 years.

Now, the imprisoned fighter -- real name Jon Koppenhaver -- is speaking for the first time from behind bars as part of a documentary by Jon Bravo ... and he's expressing serious remorse.

"There's not a day that goes by that I don't regret that, and I hate it," he says.

"Nobody hates me as much as I hate the man that I used to be."

Machine says he initially blamed Christy for the incident because she was cheating on him -- but has now realized none of it was her fault ... he was the bad guy, period.

"From the bottom of my heart, I apologize to Christy. I'm sorry, Christy."