Next time you ride, drop your stirrups and ride for a few strides. Allow your leg to fall where it is naturally comfortable. Then stop and look down. Is the stirrup hanging just by your foot or ankle, or is it hanging in front of your foot? Is it hanging too far below your foot? It it falls too low your stirrups are too long. If your stirrup falls in front of your foot when your leg is comfortably beneath you, then you need to switch saddles or add a riser pad (though changing saddles would be the ideal solution). If this is not the case, then it might be a muscle or balance issue on your part, or it might be the seat of the saddle. Do stirrupless riding to build strength and balance, and try different saddles to see if your seat improves.