Thats just natural, especially if you have been cheated on before. My ex wife cheated on me for years before I could catch her, so here I find myself waiting for Leisa to cheat on me, even though she never will, I just assume she will because of what my ex wife did. You just have to work on getting over it and realize you cant assume the worst in someone or they will cheat because they figure if you think they will, they might as well do it.