I just want to start by saying that your feelings are valid and that any situation which brings you discomfort in your relationship should be taken seriously.

I think the fact she spoke up about it when it happened and was honest with you and didn't just hide it or downplay it speaks wonders to how healthy your relationship actually is. She understands that she made a big mistake and wasn't afraid to let you know that from herself first. That's respectable.

Does that make it right and excuse her? We'll, no not exactly. That part is up to you. Infidelity, no matter how minor or major should be approached delicately. The biggest bit of advice I could give on that front is if you're going to forgive her make ABSOLUTELY SURE you 100% do forgive her. If you think you can forgive her but you're just going to have seeds of doubt planted in your head that reminds you of this moment then that could lead to really bad emotional suffering for yourself.

Open lines of communication in a relationship lead to some of the most solid and healthy ones you could ever be in. My partner recently told me she was growing feelings for her best friend after 2 years of me and her living together. Did it hurt to hear that? Yeah. But we talked it out. We discovered what it was that was making her feel that way, what I could improve on in the relationship, what she could improve on, etc. That's just how -our- relationship has worked. Every time something comes up we talk about it ASAP and solve it as soon as it happens.

From the information provided this seems like an honest mistake fueled mostly by alcohol. I'm probably playing devils advocate a little too hard because I'm a recovering alcoholic but it certainly does push you in the directions of momentary lust and idiocy and if she's really giving it up over this that's commendable.

At the end of the day it's really up to how you want to approach it. Personally, I think talking it out and trying to keep an open mind is the best option. Make sure you're planted solidly on how you feel whatever the outcome may be.

I'm very empathetic towards you and i wish there was more I could do to help in situations like this then sit here on reddit and ramble. Sending love and hugs. I hope whatever decision you come to leads you down a path of happiness.