She appears to have a pretty high natural head set, so I'm not sure how "low" she will be able to go physically. However, that does not mean she can't do dressage, even with a naturally higher head set; she will still be able to collect.
I think she's a very lovely girl, and once you get her to relax, she will look even better, because she will look happier. Just keep doing what you are doing with her; desensitizing her on the ground, working slowly on relaxing undersaddle. One thing that helped my guy relax was to take him on walks, just to let him graze; after few regular walks in which I lead him, I started climbing on, and still, just let him graze. After a while I would ask him to go a few yards, ask for a halt, give him a cue word 'okay', and let him graze some more. A horse who is too nervous will not eat, so if you can get her to a point where she will, you are getting somewhere. I don't have to graze my guy anymore, as he now is pretty relaxed undersaddle, but occasionally, I will stop and let him eat, just to remind him that being ridden is still a 'pleasant' thing. Because I have 'controlled' when he eats, he doesn't try to on his own, either.
"The ideal horseman has the courage of a lion, the patience of a saint, and the hands of a woman..."