I know your feeling may impact your ability to see things rationally for a while, but I'd like to present a counter-point for you to consider when you're ready. You think it's your fault for providing him the opportunity to cheat. Have you considered that maybe he was always a cheater and that has nothing to do with you? There are plenty of people in committed relationships that have plenty of opportunity to cheat, but do not because they do not want to. The opportunity to cheat isn't the only variable in this equation.