My (30F) girlfriend got an affair with her friend while I (29M) was next to them.

I've been close to 1 year with my girlfriend. Not long ago, I setup a pool party for her birthday with all of her close friends and at the end of the night, mostly everyone leave except 2 of her friends. We were chilling in the pool and couple minutes after she start kissing one of her friend passionately for a long time and start fingering and eating themselves. I try to come close to participate but they totally ignored me and I just leave and come inside the house. I talk about it with a friend and I didnt know how to react. After 20 minutes I just step out and ask her I was about to end our relationship if she continue and I leave the party to go home.

I don't know how I should react. The next day she said she was sorry, was crying and told she was drunk and couldn't control her. She said this won't happen again and that she love me.

This moment still haunts me and I don't know if I should just leave her or get over it. I still love her but I needed to talk to someone of this.

What would be my next option at this point and how to get over it to forget about that?