When I talk to him about it, he dont understand why it is a bad thing to be good friends with his ex.
It may be a "bad thing" from your point of view, but a friendship between parents is the best possible situation for children, whether the parents are still a sexually active couple or not. You claim to agree with this, but in truth you hate it.
Your blindness to the needs of the children, along with the rest of your misunderstanding of the true situation of these people, results directly from your craving to have things be as you want them to be instead of simply seeing things as they are: a man who loves his children, loves his wife, and never really wanted a divorce anyway. You are there for sex and forgetting his pain, never to replace the people he loves.
Your focus on your own desires, and complete ignorance of the actualities involved here has brought a false self (a false "you") into existence--a "you" which would not be there if you saw the big picture instead of focusing entirely only upon what you want, and imagine you need. It is this false self which now suffers the pain so evident in your post.
As the guru, Nisargadatta, used to say, "No wonder you are unhappy. You are thinking only of yourself, and that doesn't exist."
Yes, You are absolutely right, I do want things different... What woman wouldnt. That is not being selfish. The thing is, he told me he was falling for me and wants to be with me. Then he tells me that he is starting to hate her. So your in love with her or you hate her, which is it. And as far as the kids go, I am concerned very much for his kids. His kids are way too young to understand what is going on at ages 5 & 8. There father is living one place, while them and the mother is living another place. We do things together as a family, so hmmmm maybe mom and dad are going to start living together again. So many questions those kids must have. It is one thing wandering what divorce is, but really confusing them about still spending time together as a family. And... No, i do not only think of myself. I left him so that if there is a chance for them to get back together, I wont be standing in their way. Plus it gives me the chance to find what I am looking for instead of wasting time on something that will never happen. If that is selfish, then... Whatever. But thank you for your reply!