Try turning your toes in a bit more I assume.. that will help. Also keep your heels down and leg right behind the girth.. but no stirrup will force you to keep ur leg there so even tho it sucks it is super beneficial!
Put your phone between your leg and the horse. Works a treat
Try no-stirrup work to help build up your leg muscle, or even bareback if you are comfortable enough. And sit up straight and deep in the saddle