The first post said nothing about abuse, however I stand by my post. I work with a lot of abused, neglected, and feral horses. It doesn't have to be caused by what you are doing, but more caused by what you aren't doing.
You stated that she has been getting worse, and this means that she is currently missing something that was previously there. I realize it is a process that takes a while. I still say that you need to set firm boundaries with her even if she is skittish and this will give her faith in you as the leader.
Some people think that a horse that lacks respect for a human is running them over or showing aggressive behavior. However, that same lack of respect can be shown by a horse that is spooking or showing anxious behavior. Giving a horse the label "well, they were abused" is the worst thing that you can do with them.
If I turned your paso mare out with my herd, she would be given the cold shoulder, they would not even try to interact with her and would actually push her away. They would not try to shelter her and they would not baby her because she had it rough, believe me, most of them came from the same situation.
Remember, your horse has no idea she was abused, she remembers experiences, and she may associate certain actions with people or pain, but she does not sit in the pasture and dwell on her past life.
Look at an abused child. The abused child tends to turn defensive, the best thing for them is not spoiling and giving them what they want, but they do best with love and good, fair leadership. They need to know where the boundaries are because that is where they will begin to find their confidence. Once they are comfortable, those boundaries can be extended.
Horses come with baggage, I am fully aware of that. As the handler, you can let them carry their baggage, show them where to go, and deal with the baggage as it interferes. Or, you can be the leader, carry that baggage for them, and have a horse that trusts you completely to the point where they check with you before they react.