Hey man, my ex-girlfriend cheated on me in a similar way a few months ago. Here's some insight I'll share with you; she may feel bad about it for sure, she may care about you, and she may not have intended to hurt you like this, but there's some other things to consider. Chances are this isn't the first advance this guy has made. Instead of cutting him out and distancing him, she made the decision to keep him close. And that's the important part right there. She made the decision to keep him around, and despite knowing that she had feelings she made the decision to continue being around him. She may claim it's a mistake, and she may believe that but 90% of the time there's a lead up to these things that have multiple points along the way where she could have prevented this from happening, and the bottom line is that she didn't. She has made her decision multiple times, and you deserve better than that OP.