And it seems crazy to me to throw away nine amazing years

If it's been nine amazing years for her, she wouldn't be doing this. But she is.

This sin't one weekend, dude. You know it's not. She's been letting this guy pine for her, stringing him along, and PURPOSELY getting closer every time. She has been making these choices over and over, and not just this weekend.

People change. And I understand clinging to what you thought was good and right -- but that's not what the reality is anymore. You can't cling to a ghost, hoping it comes back to life. Your girlfriend has chosen to be a different person, and bending over backwards can't bring back a part of her she's decided to get rid of.

Your "perfect" relationship is over. You will never be able to forget how she hurt you, nor will she ever forget that you basically just let her cheat. She'll keen this in min as she looks for other interesting partners.

You can have nine years with someone who GENUINELY will be happy and loving with you, not secretly wondering "What if?" and then going off to date another dude right in front of you. You could ACTUALLY have that, instead of trying to force reality to change to a version where she isn't blatantly disrespecting and cheating on you.

But you won't as long as you stick to a person who clearly doesn't respect you. There's very few things a person can do to disrespect their partner more than blatantly date someone else right in front of them. If you forgive her, there's no going back. You'd be taking on pain to try and bring back a relationship you can never, ever have again, nor had you genuinely had for a while, anyway.