How can I get steadier legs when cantering?

My instructor says I have a very steady leg when I trot but when I canter my legs move a lot and I can't keep them still. It's completely fine when I canter horses with a smoother canter but on certain horses, I can't keep my heels down and my legs sort of flop around. My instructor keeps shouting at me to push my heels down and tighten my legs and I can sort of do it for a few seconds and then I can't. She says the only thing I have to change when I canter is that I have to tighten my legs. Today I even lost my stirrups and cantered a lap without them, which I actually found way easier. Does it get better? I really want to improve. What are some exercises I could do at home to get better at it, to have a more steady leg and be able to keep my legs tightly wrapped around the horse when I canter?