Posting without stirrups. It sucks but helpful.
Trotting around in jumping position can help strengthen those muscles and get your heel down. This is a good start. Next, I recommend sitting the trot. Finally, posting. You don't want to stand straight up or post on your toe. You're going to want to rock your hips forward in rhythm to the horse's movements while still rising in and out of the saddle. Make sure you're still over your horse's center of gravity! Hope this could help! Good luck!
This is what I did and it worked for me so feel free to try. start trying to keep your feet barely touching your stirrups and post by using the muscles in your thighs and core. By doing this I went from not placing at all in equitation to a nice 2nd place in 2 months. Your legs will be really sore probably and it might bother your back at first if you have back problems but after the muscle is there you will feel like you could post correctly all day :)
Get someone to lunge you on your horse and put a whip behind your arms and it helps alot
Strengthen your legs. Yo can do
This my standing up I. Your stirrups without resting your hands on the horses neck for a couple