Beth, I have an idea, something that I actually CAN help you with; the backing! If she resists, "over rounds" or threatens to rear, the she feels stuck. I will take a guess in that perhaps you are asking her to back using both reins, right? Both reins on , and evenly so, at the same time?
Here's a thought;
Next time you ask her to back, first of all really look for a change in her thinking. I mean, look for her thinking about backing, and give your release when you feel the slightest change in her body as per her thinking about backing. ALSO, put a little bend in her first. Get her to give one side of her face , I mean give at the jaw, and then ask for the back up. To do this you will have to pick one rein, apply it actively (like a soft "milking" of that rein) until she gives in that jaw, then while keeping that bend you bring in the other rien to say, "back up" But keep the bend createing rein as your dominant rein.
If she really locks up and starts to feel like the only way out she can figure is UP, then drop the supporting rein and take the bending rein in enough to actually cause her to disengage the hind quarters. She;ll step that side hind leg under her body and rock her weight back onto her hind , then she;ll straighten out. And you try again with the back up. The purpose to doing that little disenagagement of the hindquarters is to break up her brace, get her feet moving and then she should be softer and more willing to back up.
And be sure to accept a small try at first with a big Thank You on your part.