Walk up stairs with your heels down and it should help
Comedown and relax. Then more you relax it's getting easier to swallow down and around the horse.
Put all of your weight to your heel rather than the ball of your foot. Practise this while flatting and you will get the hang of it in no time.
one of the things I try to do is on flat days ride with shorter stirrups so my heels can feel what it's like to be pressed down and stay in position , then on jump days have my stirrups longer and practice keeping my heels feeling like they do with short stirrups
What I do is I put all my weight in my heels! Also, strech your heels before you ride by going to a wall and putting the balls of your feet on the wlall and push down on them!
Walk around on your heels
Try this: Stand on your stairs with the ball of your foot on the step. Stretch downward. It stretches out your calf muscles so it's easier to step down into your heels in your stirrup. Also riding with your stirrups 1 or 2 holes shorter makes it easier to practice stretching down into your heel.
Practice riding without stirrups and think about stretching your leg long and down and sitting deep to encourage the weight to come down into your heel.