"I’ve been doing horrible and I realize that now and I’m doing everything I can to change it."
That's "Mayhem" Miller getting real with his thousands of followers ... sharing he's been seriously struggling after years of bad behavior that often left him in handcuffs and behind bars.
But, the 42-year-old retired MMA fighter is vowing to change his life for the better.
"I went away for a couple of years to pay my debt to society, and when I got out, I realized that I have much more paying to do," Mayhem wrote on social media.