My (20M) girlfriend (20F) said I was being unreasonable by not wanting her to go to the party where her ex will be there after I told her if she goes I would most likely not be her boyfriend anymore.

So my girlfriend and i have been dating for 2 years. We decided to break up for the summer since I was going away and she was staying home. We decided to stay friends and we remained friends the whole time even talked often. We got back together when I got back and everything was great. A week ago she told me there was a party coming up and she wanted me to go with her. She said she wasn’t going to go if I couldn’t go but then I found out her ex was going to be there. I told her I didn’t feel comfortable with going to the same party as her ex and she let it go. After some thinking she came back to me and told me she wasn’t going to the party and that she was going to tell her friend. I said it was great but then she said she decided she was going to go. I asked her why and she said she didn’t want to miss out since all her friends were going. I was frustrated and told her I wasn’t going to go and she started complaining how I was letting her miss out when I didn’t have to. I told her if she went we wouldn’t be together. And she said yeah I guess you’re right. The next day she told me she wasn’t going and that she told her friends and they understood. But the next day she told me she decided she was going again and she was only not going because of me and she told her friends again. I was irritated and she told me she was either going to go to the party or break up and I told her it was her choice. I haven’t heard anything from her since not even a text. I don’t know how to feel, I don’t want her to miss out but I don’t want her to be around her ex since they were in a relationship that lasted for 3 years. I feel lost and I don’t know what to do, please help.