12:50 PM PT -- Before he left for rehab Thursday, Xan came on "TMZ Live" to explain exactly why he's going, and it's interesting ... he says he's nowhere near "rock bottom."

He told us he's been far more strung out on opioids in the past, but feels he needs treatment now to get him "over that last hump." Perhaps not coincidentally, he's entering rehab on the 1-year anniversary of Lil Peep's fatal overdose.

Xan also broke big news about his plans when he completes his treatment. He's making at least one huge change.

Lil Xan is dead serious about kicking his drug addiction because he's headed to rehab ... something he hinted at when he spoke to us earlier this month.

The rapper announced Thursday he'll be checking himself into a facility by next week, and said he's looking forward to being clean so he can finish his Mac Miller tribute album.

As we reported ... Xan reached out to us when we broke the story about Mac's cause of death -- cocaine and fentanyl -- and he opened up about his current struggles with addiction.

Xan said the death of his hero, Mac Miller, coupled with the death of Lil Peep almost a year ago, really opened his eyes. He said he'd curbed his drug use, but still wasn't clean.