Boss rant

I need advice I how to tell my boss I want to switch departments. 
I've been a coordinator not officially but basically Bc our first coordinator up and left with out saying a word. So I jumped in Bc I had no choice to. Keep in mind I was doing all of the work and I wasn't getting the benefits. Just recently my boss told me I'm not not "meeting her standards" I put my heart and soul into this job. I can no longer take it. I think it's just best if I switch to a different department. But I don't know what to say to her about doing so? Can someone please help me on what to say to her?