How do I deal with the heartache?

my good friend at school, who is a girl, used to hang-out with me all of the time, now, she has a boyfriend, and she completely ignores me. I tried saying hi to her today, and she ignored me, and went on with the conversation with her boyfriend... I am really upset at the fact I did not tell her I had feelings for her, and she was "taken" from me by another close friend. What do I do?