If your horse is heavy on one rein, and empty in the other - fight your instincts to take back on the heavier rein. It won't help. Instead focus on engaging the hind leg on the heavier side to put the horse more up into the opposite rein.
In the sport of Eventing, ensuring that our horses are truly fit for our level of competition is one of the best ways to prevent unnecessary injuries.
Horses become incapable of learning when they go past a certain level of fear, anxiety, or stress. This means that if you keep hammering away at the horse, trying to teach them something when they are currently overwhelmed with stress, it will not work.
For the horse, the learning happens on the release of the aid. That means if you don't release the aid, the horse will not learn.
"Don't take the tempo down without revving the engine." ~ Chris Bartle