She's cute, but like others said, the price tag is too much for a horse like her. In my area, you could maybe get $600 for her. Possibly $800 if she has superb bloodlines. As for her general conformation, her back is long, she's fairly downhill, and I really don't like the look of her front pastern angle. For pleasure riding, it would be fine, but for anything requiring performance, there is a good risk of injury there. Too long and too much angle.
As for the bracing and head slinging when you apply pressure to the reins...have you had her teeth checked? It strikes me that you are a bit quick with your hands and your release is timed wrong. That may be why she's not a supple as she could be.