Any major experience changes a person. It gives you a new perspective and makes you think differently about yourself. Heartbreak is a big deal, and it makes sense that it can take people a really long time to get over it.
If a relationship was really significant or long, it might change someone even more than if it was less important or shorter. Usually people change for the better after heartbreak, though, because they have learned something about themselves, and about other people. You can go through a fruitful article posted on Go Marry for more information on the pitfalls of heartbreak.