That is a good feed with 14% NSC (low sugar/starch) and high protein. I assume your horse is not overweight or underweight? If she is underweight, then lack of energy would be from lack of calories, not lack of sugar. If she is overweight, that makes a horse have low energy too.
It is unhealthy for horses to have high NSC levels in their feed. So that is not the answer to your problem. I have some very high energy horses that had no change in their energy level when I switched them to low NSC diets. Sugar does not give horses long-term energy. That comes from calories from fat, protein and carbohydrates.
Since she was a high energy horse in the past, I would assume something is bothering her. It may be your saddle fit, bitting and/or mouth issues, sore back or sore feet. She may have bonded to horses she is turned out with and is displaying unwillingness to work because of separation anxiety.
There are so many reasons this horse may have become unwilling to work. You need to be a detective in order to figure it out. Are you drilling her too much and making her bored? Are you rewarding her appropriately and giving her time to rest? Are you over training her so her muscles are sore? Does she have a new farrier and her feet are sore? Is your riding balanced and kind?