I'm mad at my brother's girlfriend that used to be my best friend
Why are you upset with her?
She claimed that I never liked her. And she's wrong!
You should stand up for what you believe to be true. You should explain to your friend what you feel about her deciding to date your brother. Doing this will help them to understand your perspective.
I agree it's wrong for her to use my brother.
She is able to date who she wants to date, that is something you cannot control. If you feel that she is not appreciative of you, you can tell her how you feel and hopefully solve it that way.