I'm always. Unhappy and I dont what to do what to do. Plz help!!!!?

Latly I've been feeling majorly unhappy... So this girl that was my best friend is now talking behind my back and nevr talking to me and cuts me out of conversations and everytime I talk to someone she just but in an start a hole new conversation wit them. She always has to do better than me and try to make me jealous( which I am a little) I'malway there for her and she is never for me. Shes talking to the "populat ppl" we always said we'd nvr try to be popular but now she is trying, she always acts stupid and never takes thing seriously , she ignore me and is trying to make my life horrible, I've tried talking to her but she doesn't care, and I feel she's taking all my other friends . Latly I haven't been playing well(cause I play soccer and tennis) and it takes me forever to fall asleep, I'm super hungry and I have lots of homework (cause I'm in all pre-ap cles, exile, honors what evr u cal it) and Latly i haven't been turning in my work lAte when I've always been a good student, it just seems like everything in my life is going rong And I get off track a lot and can't focus on things and I want to be alone all the time ,do u now whAts Wong or what I can do to make my self feel beter... Also I don't feel like I have any self confidence & Im always tierd... Plz help!!!!