As I read through this thread, I was thinking that this horse is not coming up under herself well enough.
The other thing that keeps coming to mind is that it takes two to have a horse brace. If you are not accommodating her, she cannot brace by herself. She is bracing against your hands.
You need more true collection and I like to see more 'self carriage'. If you try to 'pull' a horse together, you will get a horse that braces. If you 'push' a horse together, you gets its hind end under it and it won't brace. Another thing I like to see is a horse that can bring its hind quarters to the inside. This so much more effective and helps later with more difficult maneuvers. Leg yielding exercises cause many problems when used past a very green horse just learning to get off of a rider's leg.