Forget it; why beat yourself up with thinking about the past. Will it change your life if you know this answer? No, because she's still not with you, and she'll cheat on you again if you two get back together. Get a hold of yourself, have some self-respect and conviction and move on.
Is this what you really want? a cycle of endless heartbreak? woth all honesty this. doesn't sound like love to me. sounds more of an obsession. you can do much better then her because you deserve better.