Honestly i think its a bit weird that she left you to experiment with bisexuality. it sounds to me that she is unsure of what she wants. i think that if there is still a possibility of her being interested in girls you should tell her that you dont think you should be together until she is sure about what she wants. i also think that it was wrong of her to cheat on you.
All sexuality aside, i think that your relationship is too complicated. you have been on and off for three of your high school years... you need to meet new girls instead of limiting yourself to just her because i can assure you that there are more girls that are actually worth your time