I...actually very recently went through something quite similar, though in my instance, it turns out I was the other guy...never even knew. Good God does it hurt.

I offer that to tell you I do understand how you feel. It is incredibly hard. The fear of it happening again, I think, is going to be a constant factor until you two really sit down and talk about it. She cannot rightfully expect you to brush this off. She needs to understand that while you're trying to work through this, she has to help you.

I would recommend you two sit down, and explore this from every angle. Why she did this, how you feel, how she felt and feels...everything. Only then, I think, will it subside.

I offer you my PM box if you need me, or any perspective I can offer. I wish you my best.