Cute horse! I have a weakness for buckskins.
You do look tense, but who wouldn't , knowing that they are being videotaped. Your arm IS stiff but at least your hand maintains a fixed position , relative to the horse's motion, so that the rein isn't snapping tight, loose, tight, loose, as some folks do.
The problem is that you are posting form your KNEES! Your body weight has stopped at your knees, therefore your entire lower leg is ineffective and the heel comes up. AND, when you post solely form your knees, your energy just kind of rigidly stops there and it puts you in a fragile position; if the horse "propped" (stopped suddenly) you would be thrown forward off the pivot point of your knees.
So, get your weight flowing down your thighs, through your knees into the ankle and heel. Think of your heel being kind of focussed at your horse;s hind feet. This will help keep the energy down and back. Also, think more of your pelvis coming FORWARD rather than so far upward.
If you pick up your horse's trot so that there is a bit more forward energy it will be easier for you.
You will be a really cute English pair!