It sure sounds like you know what you have to do, just having some trouble getting there. Any chance of some meetings or maybe some talks with a good counselor (probably someone who understands addiction and codependency)?
Why do you think you feel guilty? As you have said, you are not the one in active addiction; you did not leave and move in with someone else; you did not alienate yourself from your children; you have stuck to your end of the bargain and then some. You have loved your husband unconditionally and now you have a right to move to a new place in your journey. Do you feel somehow if you were different, better, more worthy that he would not have left and chosen drugs over you? I hope not, dear one because you are so worthy of more than this and you deserve only happiness in your life.
I hope you can visualize letting him go, releasing him to a power greater than us and taking the good memories of your marriage and storing them in a lovely chest to take out and examine every once in awhile.
Wish I had something inspiring to say, but I don't. Just adding more hugs and lots of prayers.