Michael Phelps -- I Felt Like I Wanted to Die ... After DUI Arrest

Michael Phelps says he was "in a really dark place" and didn't want to be alive anymore after his 2014 DUI arrest ... but says he's cleaned himself up and insists he's swimming better than ever.

The Olympic legend -- as well as his close friends and family members -- opened up to Sports Illustrated ... and painted a picture of a man who had a serious alcohol problem.

"Honestly, I thought, the way he was going, he was going to kill himself," Phelps' longtime coach Bob Bowman said ... "Not take his own life, but something like the DUI, but worse."

Eventually, Phelps was convinced to enter rehab in Arizona -- where he spent 45 days -- and believes he's changed.

In fact, Phelps says he's sworn off booze until at least after the 2016 Olympics in Rio -- after enduring years of working out with a hangover.

"Haven’t had a single sip and will not have a sip. My body fat has dropped significantly, and I’m leaner than I’ve ever been."