My girlfriend wants to break up with me because of a joke I played.

Emotional things, sometimes you will feel very complicated. For example, if you are confused by a joke that she wants to break up with you, it's actually very simple. A relationship will not be separated because of a joke. I think there are two possibilities. First, you have accumulated a lot of problems since you fell in love, but this time you broke up because of something. Second, she has a new lover, just using this joke as an excuse, as a fuse, as a reason to break up, making you feel that breaking up is because of you, so she will blame you for breaking up. The two possibilities are 50/50 each. Although I don't know what joke this is, it is ridiculous to break up because of a joke, and this emotional foundation is not solid.