My girlfriend is bi and she is in love with me and another girl What do I do?

Basically this is a yucky situation. The fact that your girlfriend is bisexual is irrelevant; if she were straight and were in love with your male best friend, you'd be in the same boat.

It sounds like you are not okay with the situation. Some relationships might be negotiated to allow this kind of situation, but you are obviously not open to this situation. You simply have to tell your girlfriend that you need more of a monogamous relationship. It's okay for you two to love each other but also decide not to stay together.

If your friend has expressed interest in your girlfriend back, then your friend has betrayed you. If she has not, or doesn't know, then there is no need to raise any of these issues with her. If she has, you should tell your friend that she has betrayed you (how and why), and that you are mad. Whether you two can reconcile depends on where you go from there and how you feel.