1) You learn how to spot red flags. There were probably some signs at the beginning that you were ignoring, and you probably won't make that mistake again.
2) You learn what type of personality doesn't match yours. In most cases our exe's were not "bad" people, they were just not right for us, and it's important to realize what type of personalities match best in relationships.
3) You learn to be strong. Going through a bad relationship certainly isn't fun, but it does make us stronger if we are able to learn from it.
4) You learn more about what you want in a partner. We don't always know what we like and dislike in partners until we live through those experiences. Look at the bright side, at least now you know what you don't want!