My husband's Anglo Gelding did similar. His was a combination of two problems: pain and his diet. He had pain in his hip from an old injury that was causing him to act up at certain times. Since he was thin and a "hard keeper", he was getting a LOT of feed. Once we addressed both issues, his attitude changed 200%.
So, I would first look at her back health and saddle fit. Even if she acts fine for tacking up, she could have some pain issues from the saddle. Have a good equine chiro out to evaluate her.
Next I would look at her diet. Many TBs are sensitive to corn, oats, molasses, and alfalfa. I would have her on a diet of only grass hay or timothy and NO GRAIN or pelleted feed. If she needs extra, I would give her some soaked shredded beet pulp (the kind without molasses), a ration balancer (like Purina Enrich 32 or Triple Crown 32% Supplement) or vitamin supplement for horses on no grain (like Select II or SmartVite Maintanence Grass), and some stabilized rice bran (for added fat). You can feed lots of the beet pulp (it's not a "hot" feed) and up to 2 lbs of the rice bran. All of this is a "cool" diet. Switch her over slowly, and give this diet 30 days before passing judgetment.
Nutrition plays a LARGE role in horse behavior and is often overlooked. Also, horses deficeint in Magnesium and B-Vitamins can be "spooky."
In the mean time, you need to work with her on the ground. Instead of riding her on the trails, take her on "walks" on the trail, like a dog. Just you and her. Work on leading her from both sides, trotting in hand, standing quietly, etc. Let her graze when she's been good, making the outing "fun." Walking in hand away from the property is a GREAT bonding experience and will get her to turn to you for comfort, rather than another horse.
In the round pen, work on lots of changes of direction, transitions, "WHOA", etc. Keep working her until she shows obvious submission (head low, chewing/licking, an ear always on you). Then let her rest. Keep her in the round pen. Once she's cooled down, offer her a drink and some feed. Feeding her in the round pen will make it a "fun" place, not just a work place. Make sure to always praise her when she's doing well, with a "good girl" or even a treat. Most of the horses I have worked with responded very well to lots of praise when they got things right.
When she acts up, try just making her stand still. Make her stand until she's calm and quiet, tell her good girl, then go on with what you were doing. Some horses do not respond to the method of moving their feet when they do something bad. The other alternative is to make them stand quietly. Give her time to think about things and quiet down. Then go again. Rinse and repeat until she does it right. If you remain calm and quiet, she will eventually get tired of this "game" and do what you want.