Do you have an independent seat? Then try this exercise.
Move your left arm up and down while rotating circles with your right hand. Can you manage that? Very well, then move your left leg up and down while rotating your right foot. If you can do that then try alternately touching your nose with your right hand and your left hand while rotating in circles with both feet. If that goes well you are really very good in controlling your body, because hardly anyone succeeds. The most difficult thing to do is doing circles with your left foot and going up and down with your right leg.
Good riders are always working hard to improve their posture. A whole hype has arisen around equestrian fitness, with the help of famous Dutch horse people like Lammert Haanstra and Adelinde Cornelissen. In addition, there are now flex chairs and rein tension devices to make you more aware of your influence on a horse. Very good, especially for the people who spend many hours a day in an office chair. I have horses at home and taking care of them is already a fitness program in itself. In addition, I am an agile type and I run, because I think it is important to be in such good shape that I never run out of breath on a horse. Adelinde is always good at explaining why she is a fitness freak and says it creates time in her mind to think along the way if her body can easily handle the physical effort.