Feed may be affecting her energy level, but should not be a factor in her manners. I rode Thoroughbreds for years (in training for racing and as hunters), and they were none of them unmanageable or particularly hotheaded as compared to other horse breeds. Yes, they had a lot of energy, but if it was a day for jogging, they jogged. In addition to reevaluating what she eats, I would go back to kindergarten with her, and work on manners. You might need to shorten your training schedule, and quit after she has done something correct. When she learns that stopping is her reward for listening and obeying, she should improve (one would hope). How long have you been working with her, and what was she like before you started? Is this new behavior for her or an ongoing issue? Do you do more or less the same things in the same order? Try changing, and confuse her. If she doesn’t know (or think she knows) what you want to do next, she will have to listen and think. Work on long lines, ask for collection, etc. I’m sure you know all this, but sometimes we all need to be reminded of things from one horse to another.
It is never the horse's fault