You’ve had some great advice here, especially about saddle type and stirrup length.
Im no expert but one of the best pieces of advice I can give is to stop trying so hard and don’t overthink the whole thing. Focus on your upper body posture, not the posting itself. If you keep yourself in correct ear/shoulder/hip/heel alignment, you can’t be tipping forward and sitting on your crotch.
Instead of trying to force your heels down (which makes you grip with your legs) think about just letting your weight fall into your heels and keeping the stirrup on the ball of your foot when you post. You can do heel stretches like standing on the edge of a stair and let your heels hang down and that may help, but some people just aren’t conformationalky suited to have that extreme bend in the ankles.
When you do post, think of it as a forward movement with your abs / hips, rather than an upwards one, letting horse gently push you.
While you do want to keep your legs relaxed, you need a bit of tone in your upper leg / inner thigh to control the downward phase of the post, rather than thumping back in the saddle. If you focus more on landing softly than on riding out of the saddle, you may find it helps you too.