A rep for Shifty -- real name Seth Binzer -- tells us, "Seth has completely changed his life around and has checked into a highly regarded drug and alcohol treatment center where he will be working with them to restructure his life over the next 2 years."

As we reported, Shifty was sentenced yesterday to three years probation, stemming from his February drug arrest -- when cops say he roughed up his ex-GF at a Ross Dress for Less while carrying rock cocaine. Shifty was also ordered to stay away from his ex.

But Shifty's extremely remorseful over the whole incident, telling us, "I would like to sincerely apologize to the parties involved ... I feel very bad for those who I have offended and those who have endured this situation."

According to his rep, Shifty's been in drug treatment since April -- having immediately sought help following his release from an L.A. hospital where he was treated for a mysterious coma.

We're told Shifty -- known for his multiple stints on "Celebrity Rehab" -- just got off tour, and has even quit cigarettes cold turkey.