I want to go to my fraternity's retreat instead of going on vacation with my girlfriend.
Can't you go to both?
No. They are happening the same weekend.
Both options sound fun, so it is difficult to chose which one to go to. You should carefully consider both options because people will be mad at you no matter what you chose.
I think I'm just gonna break up with my girlfriend and go on the retreat.
I think it's fine to choose to go on the retreat instead of vacation with your girlfriend, but breaking up with her seems like a drastic course of action. If you explain to her what you are doing she will probably understand. Just make time for you and her to spend time together before and after the retreat!