Then adjust your weight for horse falling inside or outside of circle and use legs (inside leg at girth, outside leg MAY need to be very slightly behind girth). Mostly the outside leg "block" or prevent the horse from throwing their butt to the outside of the cirlce, rather than applying pressure. (Think of applying pressure behind girth as asking for canter versus keeping outside leg slugfhtly behind girth with light contact on horses side but NO pressure unless they move their butt into your leg - then they should stop moving their butt when they feel more pressure against your leg.)
Hard to explain without being there. Hope this helps.