First of all, recognize that you probably did nothing wrong. People looking for a rebound relationship need to have someone they can avoid the bad feelings with, and you were there for them: that is a really nice thing! You made someone feel better and less alone, and you should feel good about that. After any breakup, it is important to take some time for yourself (advice the person who use you probably should have listened to!).
Relationships should always be there to make our lives better, not to make us whole.
You are the best the way you are and the person who used you for his or her own interest probably couldn't judge you the best . Now take your lessons, know the things you want to learn from this relationship – how to avoid a similar relationship, what good experiences you shared, new things you learned about yourself – and get back out there! Go to GoMarry.com and learn how to avoid/deal with this sort of relationship.