In love with me to in love with another girl in one day

Okay, so I’ve been in love with this kid for awhile I’ve liked him on and off for the past 2 years. I’m not obessed I’m just a girl with feelings. three weeks ago, his best friend told me that he liked me a lot but was confused because he didnt know if I liked him. so I made it completely obviously that I liked him. then we both liked each other over the febuary brake and he went from being a total perv and all bout getting ass to respecting me, and I was completely amazed by how he changed. I was also starting to be okay with the fact that he didnt want a boyfriend for personal reasons. Two fridays ago we went to a suprise party and everything was fine except for the part where this girl was all over him and we got into a small fight because I got jelous. then I got home from the party and he texted me and said that we couldnt be more then just friends. then monday at school one of my best friends tell me that he triied to finger two other girls but one had her period and the other said no. and now he likes the girl that had her period… I dont know what I did wrong and why all of a sudden he stopped liking mee! is there any hopee? any advice on how I can get him back?