I still fantasize about my ex
How often do you fantasize about your ex?
Several times a day because it feels like they were never mine in the first place.
It's understandable to think about your ex sometimes. However, you are expected to move on after a certain amount of time.
I just want to be with her again.
I can understand why you would want to return to a relationship that had made you happy.  However, you need to remember that your ex made up her mind to end things, and you should respect her wishes.  Try to find someone new who will make you feel that kind of love.