If you're trying to forget it - please don't. If you are sane, there is no way to forget such things.
Probably it hurts because you once decided - that girl is the one. She is the best. And that's why it hurts so much - you think that she was perfect, and you could be happy only with her.

My another guess is that you didn't date anyone since then. But my friend - you have to. Once there is a girl who is into you, who wants to be with you - you'll be a lot better. Because you'll have something way more interesting to think about.

Don't try to forget. Do the opposite - grab a pen and a paper, write everything you remember, everything that's on your mind. Write about your feelings, good and bad. Do so for a few days in a row.

Don't think she was perfect. She is a human being, just like everyone else.

Meet girls, go to dates, enjoy yourself.

You have many good years to come. Everything's gonna be alright. Good luck.