Do lots of no stirrup work!
My trainers ziptied my stirrups to my girth, it burns but really helps after a few rides!
I second riding without stirrups, however if you aren’t confident enough to do that, really press down in your heel and try your best to wrap your leg around the horse and keep your calf pressed on. Doing leg workouts at home also help tremendously to build muscle (:
Riding without stirrups really helps a lot.