Why is my friend being like this with me?

My friend found this boy on Facebook that was a mutual friend with this boy. She rather fancied him but was too shy to add him so she told me to add him first to see if he'll accept. I did and he accepted me and he wrote on my wall asking who I was. I replied saying I'll inbox him. Katie said that she was okay with me talking to him at first, in fact she was pushing me. Katie decided not to add him because he wrote on my wall (which she would find embarrassing) but me and the boy got chatting on inbox and we got on quite well. I copy and pasted the conversation to her and she decided to add him then! I asked her if she was okay with it, and she said that it was fine and encouraged me to speak more. I copy and pasted more of the conversation because he asked what college I went to and what I studied and then she went all weird. I asked what her and the boy were chatting about because he accepted her too and she just sad "just chattin ennit" Then she said "Argh, do what you want" She was being all snappy and weird... So I said "Do you want me to stop talking to him then, do you want me to delete him?" to which she replies with "Fine". I printscreened me showing that we are no longer friends, and she says "thank you" WHAT THE HELL?! She told me to add him, she told me to talk to him! Our friendship is pretty much based on her overpowering me. If I don't do what she says or she doesn't get her own way, then she is snappy with me. She makes me feel so small, and she has complete control over me. I take whatever she throws at me. How can I stand up to her? And why does she continuously get away with treating me like this? The argument was settled but this has really annoyed me and I am dying to tell her. Should I just let it go? I don't know what to do.