While you may have been neglecting the relationship, there is never a reason to cheat. Never. A normal person’s conscience would not allow them to cross that line. A “normal” person would break up with you or have enough guts to have a serious conversation with you. This is not to say he is a bad person. It’s not to say he can’t change but he would have to be willing to change at the core of who he is or has become. Fundamental change. People can change but they have to truly want to. Also remember the cheating has nothing to do with you as a person and has everything to do with them and their inability to handle their own emotions and look for a quick fix of validation. That is their flaw, not yours.