My girlfriend wants to break up with me because of my mother's unintentional joke.

I don't think it's all because of your mother. Maybe it's just a fuse She just broke up with you about this. If because your mother doesn't like her. She'll talk in the morning. Why have I spent so long with you? Didn't you notice anything unusual about your girlfriend before? If not, it only means that your girlfriend is a little petty. Girls, make a noise!