Mp, sorry you're having a hard time. It'll get better over time. I know this may sound flip but it's meant that way, it really will get better as you've done the right thing. Your ex appears to be on a downward spiral. Maybe he'll pull himself up again, maybe not, but that isn't your fight.
If it it's true he was sober for 5 years before you met then he knows how to do it. Whatever resources helped him before are still available, if and when he wants them.
Hang in there, read and post here when you're feeling down. There are lots of people who had, or are still having, the same experience. Hugs to you.