Maybe you are somehow inadequate and she hasn't been satisfied with you. Say she's vaguely brought up whatever it is that bothers her but you unintentionally never acknowledged it or took it seriously. But in her mind, she's talked to you about it, you won't change it, so she feels justified in ****ing someone else. It's pretty easy to rationalize things in a way that it works in your favor, just like you're doing. Maybe she's doing the same. If that's the case, wouldn't you rather her have broken up with you instead of taking the third option (cheating without you knowing) which is better than any other option ever!