How can I do my job if my boss doesn't let me?

I was promoted about 4 months ago to office manager when my boss became owner. She is still telling everyone what to do in the office, handling all the accounts and barking orders at me. Really, the only thing that has changed is our titles. Can anyone provide some insight on what to do? I have tried talking to her several times and she tells me that I am just being a control freak and that she is the owner and can do anything she wants. I feel I have no backing in my position and it seems silly to have two managers. I don't even want the position. I would rather things go back to the way they were because I constantly feel undermined and ineffective. Thanks for any help you can provide : )