To get the heel down I recommend to ride in 2 point position. Trot over poles and do a lot or straight lines, and try to feel that the weight is in the heel not in stirrup. To improve your sitting trot - ride without saddle( better use voltage girth if possible), but the best thing is just trot, trot, trot and trot. Trot over poles, trot without sirrups, trot with one hand, trot with closed eyes and try to feel the horses movements use your core and back to amortize the horse movements. Good luck.👍🏼
For your heels think about forcing your toes up instead f heels down, it works for me and that way your not jostling your lower leg in effort to get the heel down which is what a lot of people do:D for sitting trot relax and ride bareback a lot!
To improve your sitting trot is my best advice to ride bareback. I have ridden bare back so much in my days and it has really helped me a lot to improve my position. Especially in the trot.
To improve your sitting trot make sure you are sitting up tall and using your core😊
Stand on a step with just your toes and let your weight down into you heels. This stretches your calf and makes it easier😋