I have a five year old mare that is goofy about going down steep hills. At first she was just a little antsy when we first started down, but it's getting worse. Our trails have some very steep hills with small loose rocks on them. For several months now, she has been tossing her head most of the way down the hill and keeps trying to break into a trot. I always lean back as far as possible going down hill. I've tried zigzagging. She is shod, and I've talked to my farrier and vet, neither can find anything wrong with her. She has also started tossing her head whenever she trots or canters, even on the flat. I have always used a Dr. Cook's bitless bridle, and like I said, these problems just cropped up a few months ago. Any ideas as to what might cause this, or how I can stop it?