Before anything else, make sure she has plenty of turnout time, at least 6 hours a day, preferably 12 or more hours a day with a friend or three. She needs time to burn off excess energy on her own.
First, I would rule out medical reasons for her behavior.
Have the vet out to check her teeth, remove any wolf teeth, check her hormone levels, and palpate her uterus and ovaries.
Next, I would have a GOOD equine chiropractor, preferably one that is or was a licensed vet. Have him/her give the horse a thorough once over, including checking your saddle fit and placement.
Third, I would have a saddle fitter out to check your saddle fit and placement. I would also buy a ThinLine pad
and a real Sheepskin Girth Cover
, to make sure she's as comfortable as possible.
Last, but not least, I would look at her diet. Remove ALL grain or feed for two full weeks. Make sure she's getting only grass hay or timothy hay, or mixed hay, all she can eat. If she loses weight, supplement her with hay pellets and flax or rice bran. If her behavior improves, then something in the feed, hay, or supplements you were giving her was not agreeing with her... Keep her on an all-hay diet, adding a vitamin supplement to the hay pellets and rice bran or flax, and more hay pellets if she loses weight.