It's normal to think about your last regular partner after a breakup. Think about it. She is most of your recent sexual activity for a long period of time plus in the back of your mind is all the things you might have wanted to try with her but never did.
I have found the last one was always on my mind until there was a new last one. If you date another girl for a few months and break up with her that girl will then become the ex you think about.
Also it is our evolved tie to the the women that we break up with that keeps us thinking about them and often wanting them back. We forget the negatives that prevented things from truly working out and dwell on the positives.
It is probably some prehistoric imperative so that we would stay with our women long enough for our children to survive so our DNA gets passed along. Of course there doesn't need to be children for that pull to exist.
Functionally that is why some men stalk their ex's or break up and then desperately want the woman back. Women are more able to walk away cleanly after a breakup, so long as there are no kids to confuse the issue because their evolved reaction to a guy breaking up with them is subconsciously that he is unreliable and therefore may lack the stable parenting traits that wil allow her to pass on her DNA successfully.
Or sometimes it just means that she was extremely hot and you don't have anything that good looking handy for a tumble.
Seriously, your feelings are normal, and things will subside. It always took me six or eight months to get mentally clear of a serious relationship, but the really hot ones stayed on my mind for years. The important thing is that in the future you may think of her from time to time but it won't prevent the success of a new relationship. Patience!