Keep your heals down and ride with shorter stirru
You might be trying to grip with your legs. I would take my feet out of the stirrups and push my leg down as far as possible. Try to keep your balence like that. Then put your feet back in the stirrups and push down the same way. Try and focus on your legs hanging around the horse softly not gripping the horse.
Push down on the stirrups
I have that problem too and riding without stirrups helps me a lot. I feel so much comfortable riding bareback or in the saddle without stirrups. Try to use your legs more
Think about curling your toes around you stirrup irons