You didn't do anything wrong. It simply wasn't the right person or situation for you or her. That is part of the process. Discovering whether or not a person is right for you. It's not a matter of being wrong. If you did anything wrong per se, all you did was let yourself become too emotionally attached too soon. And, really, it was simply being caught up in endorphins -- the "high" people feel when they've met someone and have been enjoying spending time with them. That usually wears off anyway at some point and the parties involved start observing reality in terms of the type of person they are with, who they are, what else is going on in their lives, etc.

Just chalk this up to a learning experience and don't let it carry over into new dating scenarios. In other words, don't let the hurt you're experiencing prevent you from dating in the future and being scared of getting hurt. Observe each new dating partner for who they are and let them demonstrate their ability to meet your needs for dating/a relationship. Everyone gets a "clean slate".