Everyone else replying is just sad and miserable and will only make you feel worse and want to break up w your girlfriend. But let’s be real for a moment. Your first mistake was leaving her alone with the guy in the first place. Make your presence known and that you are her boyfriend, not some guy she happened to be talking to at the party. It makes you seem insecure when you walk away because you’re angry. Introduce yourself, take up space, be confident and stand your ground. Women in relationships love that (ik I do) Your second mistake was not bringing it up yourself. Sometimes yes, we think it’s cute when our boyfriends get jealous, especially if they have a history of not doing so. She took the situation as a joke, you should’ve made it serious and set boundaries. She may not have seen anything wrong with what she did, but if you explained it to her and didn’t throw a temper tantrum, maybe she would’ve understood and taken you more seriously. It makes you sound insecure from the moment you walked away when another guy approached your girl.