Michael Phelps -- I'm Going to Rehab

Michael Phelps says he's going to rehab ... this, after his 2nd DUI arrest in 10 years.

Phelps says the days following the Sept. 30th arrest have been "extremely difficult" ... and he acknowledges it's not the 1st time he screwed up.

"I am extremely disappointed with myself. I'm going to take some time away to attend a program that will provide the help I need to better understand myself."

Phelps added, "Swimming is a major part of my life, but right now I need to focus my attention on me as an individual, and do the necessary work to learn from this experience and make better decisions in the future."

TMZ Sports broke the story ... Phelps was busted in Baltimore after an 8 hour gambling binge at a local casino last week. Back in 2004, he was arrested for DUI in Maryland.

Phelps did not say whether he will be receiving treatment for gambling as well -- but it's no secret the guy has a penchant for playing cards.