I like a girl but she already has a boyfriend. What should I do?

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I am in grade 11 and she is in grade 10 and her boyfriend is in grade 10. Lately her boyfriend has stopped hanging out with her and it is sorta upsetting her. And recently my girlfriend broke up with me for my friend. not sure if that last part was relevant but i put it in there anyway. so for the last 2 days me and this girl who's bf isn't hanging out with her have been hanging out and we've found out that we've got a lot in common like: We are both English, We have the same hobbies, like the same movies, we both have a great sence of humour, we both have great personalities, good taste I fashion and cars. We are friends on facebook and her and her boyfriend aren't friends on facebook and today we just spent 2 hours non stop chatting on fb.

I REALLY LIKE HER AND I DON'T KNOW WHAT TO DO. PLEASE GIVE ME SOME ADVICE!!!!!!!