My friend, if you two agreed to be exclusive to each other, and if she is no longer deciding to do that, then you have two options:
1. Break up with her and find another person who actually does want to be exclusive. (This would be my personal choice)
2. Begin an open relationship with her, if she agrees. You are free to fool around with others now too, if you want.
Either way, I wish you the best, and I hope you learn from the relationship!