Hey. I was browing the "help each other forum" when I saw this post. Condolences about the rejection, those are never easy one one's self worth, especially if it was not feeling good in the first place.
I've dealt with it myself, and my personal advice would be to be respectful and grown about it, but to also be kind to yourself, and to not let this girl or her lack of reciprocation for your feelings be your whole world. Do not let this be something you take personally/hard on yourself, or a reason to be scared of girls in the future. Did you know her very well? Was she a friend? If she was a hallway/classroom crush, maybe you would've found out she wasn't the right match for you anyway. Sometimes people just don't match each other, but it's always possible to find the right person.
Getting over rejection and such isn't something that is easy per se, and I won't lie, it takes time. I still think about my first time telling someone I liked them and being ignored. But I only think about that time when it's a brought up in conversation as a topic, these days (which is quite rare!). You will get over her, especially if she was not someone who was bonded at the hip with you.
Spend time doing what makes you happy and fills you with passion and purpose, whether that be hanging out with friends, doing hobbies or fufilling work. Time certainly heals a broken heart and once the burn is gone, it'll feel like a distant memory. When you spend enough time doing what makes you happy, romance and love will happen upon you before you can even see it coming. Speaking from experience ;]
Good luck stranger!