Big hug to you jjnorris. This is really hard stuff and unfortunately for many of us setting boundaries makes us feel like the bad guy.
No one is the bad guy here. He is an adult and gets to choose how he wants to live and the same goes for you. You get to choose what you want in your life.
For me it was less about the drugs with my qualifier and more about him refusing to take seriously factors that led to the drugs. I cried a lot.
The best book I found for this time of grief was How to Survive the Loss of a Love. Its an easy comforting read that was written some time ago.