It's a matter of familiarity. Not only is she (I assume) your first reciprocal love, but you've spent a lot of time with her for a very long period of time. It hurts because you know nothing else but this girl. If your breakup was truly the right thing to do then you need to get back out there asap. Experience the world and all of the different kinds of people in it. Speaking from experience, it's the only cure.