I have maintained friendship with all except one of my exes. And the reason I didn't maintain it with the one is because he got really into drugs and became someone I didn't recognize. I've dated guys who I truly like as people, and who I became incredibly close with while dating. I cant seem to date anyone without becoming best friends. What I recommend to you is to just wait around for a while. If you two were close friends, then eventually you should be able to repair your friendship. You just have to want to.