It sounds to me like you're getting tense in your legs and gripping with your knees. I used to have this problem when cantering too. Really remember to relax. Sit up tall, push your shoulder blades together, and let your weight sink into your heels (don't force them down, this will just make you more tense). Think about lightly gripping the horse with your legs, like you're hugging the horse all the way around its barrel with your legs, but don't squeeze, just keep them there gently.
Pretend you have no leg and just hang on with your ankles heels DOWN. The rest of your leg should be like a wet towel on the floor, kind of sticky but not squeezing at all. Make sure you're relaxing your lower back which can be hard to do at first.
I am not from a Hunter background, so I didnt learn to ride with the intense focus on heels down. I think that sometimes folks but too much pressure into the stirrup becaseu they are desperately trying to keep their heels DOWN!
Just for yuks and giggles, try cantering and don't think a whit about your heels. Think of yourself as a legless person. Imagine that just above your knee, your leg has been cut off. YOu only have your seat bones and your thighs to carry your weight. So, when you ask the horse to canter, your knee and lower leg will cease to exist! You will LET THE HORSE CARRY YOU! Don't work so hard, just let him carry you and move you. Your job is to relax and let the movement flow through your pelvis and out your bellybutton . Look up and smile and remember, you are riding your seatbones, not your heels.
Once you are connected and allowing the horse to move your hips/waist, then you can think about your body weight flowing down into your lower legs. But to start off, be that legless body who let's the horse carry her.