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!
5) You learn more about what you NEED in a partner. On top of things we want, there are certain things we need in a partner, and everyone is different, which is why not everyone is good for us. You'll never find someone perfect, so it's also important to distinguish between things you want and things you NEED, and be ready to compromise on the wants rather than the needs.
6) You learn to let go and move on. Always keep in mind that if it doesn't work out with someone, it just means they're not the right one for you, but someone else out there is.
7) You learn about your flaws. You're not perfect either and surely throughout your relationship you had noticed some flaws that may not have surfaced before. Remember that before you jump into another relationship, you might want to work on some personal goals, and self-improvement is always a great goal to have!
8) You learn how bad your relationship actually was. It's hard to realize that we're in a bad relationship while we are still in it. Once you're out, you get to review your relationship from a completely different point of view. Take advantage of this time while it is still fresh and take in as much as you can in order to make your future relationships more successful.