Sad to report this situation is only likely to get worse over time. People who give contradictory instructions and then blame you for doing one of them are just toxic idiots who need to blame other people for something, anything, so they can feel good about themselves.

I can think of only 2 ways you might get this resolved, but both are super risky. One way is to just chew out your boss the next time they do something blatantly unfair. Light into them with a loud voice so they know they are getting chewed out.

The other way is report her behavior to someone senior to her. The thing with that is you have to be sure you'll get a sympathetic ear.

I don't like either of these two answers, but I have used both with some success (and some not success) in previous jobs. Best answer is to work hard to find a better gig.