This girl just dumped my @ss.... I feel like I can't breathe.... Feel horrible (answer please)?

It's a very hard situation. You obviously had a strong connection. It's all about establishing and maintaining boundaries. You guys crossed the line, and now she's trying to reestablish those boundaries. It hurts. It sucks. But, though I know you don't want to hear it (take it from someone who's been there, done that), you're young. Fall in love every chance you get. Don't dwell on the past. Yes, you had good times. Yes, you had great experiences. But she's telling you that her friendship with your sister is more important to her. Respect that. Move on. No one's saying you have to forget her. It is wise to sever ties for now--a clean break heals faster and easier than a messy one. Enjoy your life and all wonderful (and awful) experiences it brings.