I'm 18 we were together for a year and 2 months. She was my best friend the day she left me she agreed to date another guy. She had just met. This was prior to her leaving me. I found out today... I feel like she never cared about me... she broke up with me via Facebook... I don't hate her but I'm disappointed... I don't know what to do

Live your life and be happy

If she could date another guy whilst with you and break up with you over Facebook (seriously?), then its obvious she really isn't worth your time, emotion or love. This might sound harsh but if she could do that after a year of dating, it probably didn't mean as much to her as it did to you. So you need to do what's best for you now. Move on and be happy. Don't let her decisions affect you now that she's gone. Its really not worth it.