Get an excercise ball and squeeze with your legs as hard as you can while stunting on it. My old horse Sam used to be like that and my legs weren’t strong enough to keep him going so I tried this technique and it worked.
My trainer had me start doing calf raises and squats every night. I have really noticed a difference in the few months I have been doing it. To make it harder for yourself do the calf raises on the stairs with your heels hanging off (if that makes any sense)
I know this might sound a little childish but I always start out with marching steps (bending your legs) jumping jacks (literally Doug jumping jacks on a horse but not with your hands) and biking steps (acting as if you're on a bike) and then I take my feet out of my stirrups and two point while in a trot