Beth...
I'm glad you took the step to de-cloak and write your first post. Let's cut to the chase.
What is wrong with me?
Wrong question to ask. I don't think there's anything "wrong" with you, per se. I'm sure that whatever reasons you've had for not cutting your husband loose made a certain amount of sense at one time.
But let's look at what we know.
You've been married 20 years. Apart from a brief period of abstaining a few years back, he's used throughout your marriage. The fact that he's the father of three children hasn't prevented him from living a life of indulgence.
Based on this, the question you should be asking yourself is:
what evidence is there that he will ever change?
There is one answer to that question:
he won't.
So there is really only one other question that needs to be answered, and that's what are you going to do about it. And while I can't answer that question for you, what I will tell you is you do not need permission to do what is best for you and your children at this point. Your AH has chosen drugs. He has put drugs ahead of his marriage. He has put drugs ahead of his children. And he will continue to do this indefinitely, without conscience, without remorse. No one here would blame you if you opted to punch out of the union.
A word of caution. If you decide that, yes, you're punching out and divorcing his arse, he will promise you and your children the moon. He'll promise to quit. He will say anything in order for you to stay.
He is full of sh!t. Do not believe another word to come out of his mouth.
Keep us posted. I'm glad you're here with us.