Its okay to think about him and wonder "what if". I think that is normal.
I had alot of those thoughts also. "If only" he had made different choices, if only he had respected me and my feelings, if only he had gone for counseling when it was recommended by our therapist, if only he hadn't been only concerned about himself....then I could have had a happy marriage.
But now I don't think about the past very much, just thinking about today and looking forward to the future.