Balanced riding is somewhere between sitting and standing, but more towards standing. “Put your legs under your torso” is a common directive, which is a good beginning. Many riders make “big adjustments,” pulling their leg away from the saddle and swinging it back, to “correct” their position. Major movements do not make improvements, most of the time the leg “sneaks” back to where it came because these big movements were done with effort. When you do too much, you will not get the sensory feedback necessary to uncover the changes that need to happen. Find your balanced riding position through slow movements done with attention.
Change your reference points. Explore how the position of the pelvis/spine relates to the position of the legs. What happens in your legs when you round your back or arch your back? What happens when you lean back or forward from the hip joint? Where do you have tension? Is it in your toes, ankles, or knees? You do not have to make big movements to affect your position. Move slowly with little effort and you will become aware of the connections between your different parts. The more you can sense and feel what you are doing in the saddle, the more you can make decisions and help yourself. These movements, done with attention, will help change how you ride.
Next is to bring the torso over the legs. This is a change that occurs with little effort when the rider releases unnecessary muscular work in the hips and belly. Of course, the challenge is to bring this all together on a moving horse. Accomplishing that will bring you in the vertical alignment that helps you stay with the movement of the horse and make it easier for your horse to carry you.
Do your legs swing when riding when you post the trot or jump? The classic approach to correct this is to work without stirrups or tie the stirrups to the girth. These constraints will not guarantee you will find the solution. Stop the torture! There is merit to riding without stirrups, but when you take your stirrups back certain habits may still keep you from riding effectively. Find your hip joint, stand onto your feet (not from your feet), build the awareness to differentiate the movements in your ankle, knee hip, and learn to feel a neutral spine. Do these things and learn to stay up with the movements of the horse to avoid the struggle and develop the tools to help yourself.