After reading all these very excellent helpful suggestions, I'm thinking that maybe I'll start on the ground and have her back up from there to start with, to get her thinking about backing since she backs very well from the ground. Then when I get on I'll do some lateral movements so she's thinking about them and then ask for just one step back, then halt and go forward.
As I was thinking about this I realized that I was asking her to back "quite a bit" (just a few steps, 5 or so, but I try to do all things in moderation with her since she gets overwhelmed easily) and maybe she was just getting overwhelmed and "stuck"as someone put it.
Then if she starts any funny business I'll just ask her to go sideways, halt and then take one more step back and repeat for a few times then take a 10 minute break and come back to it, if she responded well the first time.
I'm going out there tomorrow so hopefully this will be successful. I'll let you know. =)
Isn't this the way with horses though? You think you're getting into something not too big but it's totally not what it appears. Gotta love that.