Was she a "hottie"?
One cannot expect them to stick around. How long were you with her?
The fat and/or ugly women seldom pull this stunt. They may look like dogs but they are faithful as dogs also.
It is easy to fall in love with something that looks good. I am assuming your lady was beautiful cause you seem upset by this.
BUT you should remain friends so that the other hotties will notice and you can line up another one...
BOY it is a good thing I am not a psychologist.