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.