Why cant i stop thinking about her? :(?

im 16 and shes 17. we were on and off seeing each other for about 6 months thhen we started getting into it a bit more. then she said she loved me when i tried breaking up with her in an attempt for me to stay with her. me being the nice guy i am i actually tried making it work because i didnt want to hurt her. i acted like i didnt care and i never once told her i loved her. she went to mexico 2 weeks ago and she told me she sucked some guys dick 3 nights in a row and u dont know how angry i was but i forgave her because at that moment i realized that i actually cared bout her. like 1 week after she pretty much dumped me told me she just wanted to be friends and she didnt love me and she just completely annoys me now but icant stop thinking about her i saw her today talking to someguy ( flirty way) and i almost threw some punches i was so angry.we never got to see each other much maybe once every 2 weeks but we talked every night and i actually got attached to who she was not her body and now i regret it. why cant i stop thinking about her even tho shes a hypocritical ***** that wouldnt eveen stay with me when i told her i cared for her even tho i tried my hardest to make things work with her when she told me she loved me.