That’s really tough. It sounds like you put a long of energy and love into the relationship and get little in return. It sounds like you don’t have a partner that you can rely on but another child that you have to worry about. That must be exhausting. All of his problems are becoming yours. Thats a difficult situation.
I can’t tell you what to do. But in my experience people don’t change until they realize they have to. His ADHD will never get better until he recognizes how much of a problem it is and takes responsibility.
My suggestion, you have to what is best for you and your children. Thats what is most important.
Best of luck.