She needs to learn to back a little better off of little to no pressure.
And you need to practice backing her off of some sort of platform. Maybe set up a pallet with a good sturdy peice of ply wood over it (obviously nail it on good). Start by asking her to put one foot on, and backing off, one foot on, backing off, then two, then three, then all four. Spend as much time on a "soft response" as you have too...so if it takes a few sessions to get her to step up and back off softly with only one foot, then do it...you need to build her confidence, not rush it. When she gets good at that exercise, then start all over using the trailer...one foot in, back off, two feet in, back off, and so on and so forth.
She is obviously very insecure about stepping OFF the trailer, simply because she can't see very well to judge the distance. Even with a ramp, some horses don't like backing off the trailer, because it's a small cramped space and they can't see behind them very well coming off.
"The ideal horseman has the courage of a lion, the patience of a saint, and the hands of a woman..."