Heelkat, that sounds like hell.
First off, this is his second go around with raging addiction (I take from your post today).
I'm not an addictions doctor so I don't know the difference between a pill addiction vs meth or alcohol and the different affects these might have on the brain, what I do know is they all do.
Sometimes when everyone is so focused on the addiction we can forget that while all these things damage the family, the spouse and all relationships, it also damages the addict.
Everything he has gone through changes him. He is not the person you knew before his addiction or if he has always been an addict, then in whatever he has done in the intervening years. His cheating can have no excuse and he needs to own it (but that is not your issue, that's his). Just using that as an example, according to all that's not how he really is. Well he did it and he must live with that fact now. Hanging out with a bunch of meth heads day in and day out, stoned out of his brain, that changes someone.
As for his continued philandering, in no way shape or form is there any excuse for any of that. None
. There can be no good reason, it's BS - nothing more, nothing less.
He may have quit the drugs for now but the addictive behaviours continue.
I don't know what your situation is as in if you are considering another separation, but please be keenly aware that based on what you have shared today nothing has really changed.
Addiction is tricky, while for all intents and purposes he may seem to be thinking fairly clearly/rationally, that really is not the case from what you have said. These are not "moral" failings or choices, these are the insanity (yes insanity) of addiction. Addiction, Lies and Relationships
"For in the case of the delusional schizophrenic most people are quickly aware that they are dealing with someone not in their right mind - but in the case of the equally or at times even more insane addict
, thinking that is in fact delusional may be and commonly is misattributed to potentially remediable voluntary choices and moral decisions, resulting in still more confusion and muddying of the already turbulent waters around the addict and his addiction".