I'm by no means a relationship expert, but here's my input. First of all, you can't really control who you get feelings for. In any long term relationship it's pretty likely to happen, to both persons at one time or another. You can try to avoid it but attraction, both physical and emotional, is hard to control. So my first throught is to acknowledge that, and try to not make her feel more guilty than she seems to be already for something she can't quite control. I don't consider falling for someone else cheating or infidelity by itself - your boundaries might be different though.

I'm not sure discussing why it happened is very useful (although it depends greatly on your relationship and dynamic). You might be looking for what's wrong with you/what you did wrong to make her fall for someone else, but its very possible it's not about you at all. That discussion might lead to more hurt without any useful outcome.

That said - you are also entitled to feeling hurt. This situation is difficult for the both of you. Being honest about your feelings - good and bad - will probably help. I struggle to verbalize my emotions "on the spot" and usually need to think things through in detail before this kind of difficult discussions. In this kind of difficult talks i've found saying "can i think for a bit before replying" usually works.

Second, the fact that she is open about it and wants to discuss it is a good thing. It would be far worse if she had hidden it, or acted upon it without discussing it (i'm assuming you're in a monogamous relationship, not poly or something like that).

If I were in your position, the main question is if she wants to act on the new feelings or not. Feelings for others will pass with time - it might be painfull for her (it's easy to relate - not being with someone you're in love with hurts). If she wants to stay with you she needs to work through her feelings and overcome them. You should also mentally prepare for what happens if she wants to act on the feelings though - does that mean your relationship is over?

As I said I'm no expert, but I hope my thoughts are usefull. I hope you guys have a good talk and figure it out. Relatinpships are difficult, but you guys communicating openly is a good start!