How do i know if im bi?

i have a serious bf of 11 months now and everything has been goin ok except for ptless fights and junk like that and along came a bisexual girl. We became friends but quickley started like liking eachother. I reallllyyyyyyy am fallin inlove w this girl i think and wanna be w her forever i think and we talk about doing things w eachother and etc. Shes coming tostay with me this summer and things are prob gonna go down and idk what to do about my bf who ive loved and cared about for a long timee. am i bisexual or is it just a faze??