The heart is not rational. We are totally capable of falling in love with someone who is not good for us. (I've been there and done that.) That's why love - all by itself - is not a good enough reason to stay with someone. You have to apply some rational thinking and recognize when someone is not as committed to you, as you've been toward them. You've done that, but you're still kind of infatuated with her. Time will lessen that.

The thing is you invested a year and a half in this person. That's the problem with long distance relationships. You don't get to know the other person very well. Instead, you build up an idealized image of this person. That's hard to let go of. Perhaps you met this person through the Internet. Try to spend less time online and more time meeting people IRL. Long distance relationships involve a lot of fantasy on the part of both parties. Reality then becomes disappointing in comparison.

You can get over this relationship. Accept that it will be painful for awhile. I can't think of a technique for mental processing that hurries that up. How the pain of loss resolves is kind of a mystery, but it happens.