I have lost all faith and trust in my best friend, what should I do?

First, I can't really help you as I'm pretty much going through the same exact thing. Only difference is my "best friend" didn't come out and say she didn't care, she showed it through actions instead. You're not overreacting. When people who have a hard time trusting others open up and expect someone to be there for us and they choose not to be it's like a knife to the heart. Keep your head up, you are not alone. We will both get through this!