Why am i felling sad all the time?

Im not really a dramatic person but i've ben felling sad lately like in beginning of the day i fell really good but by middle of the day i get really sad for some reason at school. im 14 btw... and if im like all really popular kids And i dont fell like im fitting in or being left out i get all mad/sad for some reason and fell like i have no freinds.. but i really do have ALOT OF friends.. but if like really close friend doesnt talk to me or somthing i get really sad and fell like i have no friends.. And this girl i like alot and she likes me to if i see a better guy then me with her it makes me fell like im not important and i will never get her but like 3 hours later i fell confident that i can get her agin? some one plz help me on how to control this cause it's happens daily and it's gettin annoying or tell me any info i need to know :) thanks!