Grow a set and either pull the plug or attempt to salvage however much of a relationship you two may have left. She may or may not have cheated on you but at least she's being proactive by expressing her dissatisfaction with your relationship rather than watching it all fall apart.
If you're generally concerned with your happiness then do something about your relationship.
If you want her then let her know that you're willing enough to fight through these tough times to give her what she wants, to express your needs and make your relationship work.
If you think you deserve better than to be cheated on then live as if you do and stop providing her with all the incentive she'd need to go outside of your relationship.
You've lost your sense of self and you've forgotten all of the responsibilities you signed up for when you agreed to be her partner. Decide what you want and then do something. It's time to
nut up or shut up.