Its your optimistic side trying to peek in on you, but mostly is that you are still used to her being around. Plenty of reason to find someone new to date which will help displace that feeling. When you have someone new to hold your attention this other one will fade from the forefront of your mind that much faster.
I know for me it helped to focus on the bad in effect demonizing her but since it was only in my mind no harm was done and it helped me past that period.