There's a phase in a relationship where you idealize your partner, because you still don't know who he is, and you project all your hopes and desires into him. But after a while, this illusion dissipates, and you get to know who your partner really is (with happy or sad consequences).

It seems in your case you never got to the second phase, so you are stuck in the idealization phase. Even worse, now that there is no relationship anymore, your imagination has no bounds left to continue idealizing your memories of the girl. The infatuation you are developing is so involved that no real girl can hope to compete against that. Similar things happen with a deceased lover, with whom the new lover has no chance of competing.

So good and a bad news to you: the bad is that the girl you are fantasying about only exists in your head. The good is that you were able experience attraction to a girl, and there is no reason you can't do it again. But you will have to see your fantasies for what they are, whims of your imagination.