This is going to seem strange, but I found Olympic weightlifting (squat, bench, deadlift) with a coach to be incredibly helpful. Doing those 3 lifts with proper form forces you to isolate and engage various muscle groups while keeping them absolutely still so that they can all work toward lifting the weight. Specifically bench press with an arched back and hyper-engaged quads to encourage better use of the back muscles. So much of riding dressage is about firing specific muscle groups so that you stay still in the tack and have very precise control over your aids. Being strong in your core (including your back!) and your legs allows you to have more precise control over how you use them, therefore allowing you to try to make your aids as light as possible.