What type of riding are you doing in the discipline of english? Show jumping, hunters, or dressage?
You need shorter stirrups for the first time sub-categories and longer ones for dressage.
To me it sounds like you are using your feet to rise instead of the horse's motion and your core.
Posting is a diagonal hip movement when you rise your seat bones come from the center of the saddle and travel diagonally upward till they are just over the pommel of your saddle (or just behind your hands)
"Strength is the ability to use a muscle without tension"