Hi, he has a problem and you have a problem. One of those problems you can solve by going to Alanon, coming here as you already are doing and by placing your focus on yourself and off of what he is doing.
You asked "How do I let it go?"
That one is easy to describe but often hard for codependent people like me. I find that I have to be ever watchful of my motives and actions.
The only way I could learn how to let it go was from constant contact with others in the same situation who were letting go successfully. Reading about it is good too, and counseling is wonderful- I've done both but only by attending Alanon and coming here to SR did I really learn how to put it action.
Recovery from codependency is possible, we are here for you and I hope you will keep coming around to share and learn. You have alot of people here who can show the way to recovery.