My Immediate Supervisor wants me gone!

Can anyone tell me how to deal with a hostile Supervisor? I am a Medical assistant/ Certified Nursing Assistant in a medical office (i am going to school for my Nursing Degree). i have tried everything I know how, to coexist with the office RN. I was a new hire as of January with little office experience, and i was quite rusty with injections and venipuncture, due to haveing to take a job in a nursing home for 6 months prior to me getting this job. I have done my best to absorb all she was teaching me, and i havent seem to pick up as fast as she would have liked. She constantly complains to the office manager that she needs to find a replacement for me and that she is tired of having to teach me everything. I have confronted her about her behavior, as well as spoken with the office manager and the physician. when confronted the RN says "oh you're just learning, these things take time, and you are doing fine." when i turn my back she bad mouths me. when i spoke to the office manager she said the same thing, and the physician constantly praises me. I dont know what to do or think. Can someone please help me?