...Bsms - I don't mean to sound snarky. But, have you ever hit your groin area over, and over, and over, and over, every time you rise in the trot for a good hmmm lets say 30 minutes or so? To the point that it hurts days after you ride. And have you rode in a saddle that killed your hips due to the twist? These are not excuses....
Nope. No saddle I've ever tried has ever caused me to post my balls into the pommel or horn. Nor did posting in my Australian saddles ever cause me to hit the poleys. If you are posting right, that should never happen - regardless of the saddle. My Martin saddle DID tilt down toward the front because it was too wide for my horse, but it didn't prevent me from posting fine.
All of my saddles would have a different twist. Posting is easier in an Australian or English saddle because of the narrow twist, but I can and do post in my western saddles as well. And ride 2 point in them.
Since I'm 57, I doubt I'm blessed with a flexible body. Quite the opposite. But if you are thrusting forward to post, ask yourself, "Why?" - because it should not be needed.
This was my daughter learning to post, at 4'10" in a 16" western saddle:
A few years later, she's 5'2", still uses the same saddle, and has no trouble posting. It wasn't the saddle, although the saddle is still too big for her. It was how she was riding and posting.
This was written from a forward seat perspective:
Bring the heels under the hip, and it applies to a regular seat. Posting should be an unfolding of your body, but your balance point should not change. If your center of gravity moves forward and back when you post, ask why - because your shifting balance will affect your horse. That is why I recommended riding two point, so you can develop a better feel for your balance in the saddle and with the stirrups.
Lunge a horse with the stirrups down. Watch what they do. That was an eye-opening experience for me, because the stirrups stayed in place. They bounced a little, but they largely stayed in one place as my horse trotted and cantered in a circle...without me. That meant my feet moving forward or back was NOT the saddle and not gravity, but MY MOVING THEM.
In my case, I found I was gripping with the knee. Not hard, but enough to put a forward/back motion to my legs and balance.
How long the skirts are is no reason to have the stirrups too long/short. I cannot speak to what would help someone do well in a dressage competition because I've never ridden dressage. But I've done enough posting and trotting in circles on a wide variety of saddles types to have thought about it. If my saddle hit my boot top, I used a different boot - or stirrup length. If I pointed my toes to keep the stirrup, something was wrong.
I won't post on this thread again. I mean no offense and am sorry if I have given any. I fully understand how the 'right' saddle makes things easier. I also suspect I may have learned more about riding while in saddles that did me no favors.
Regardless, I wish you well and hope your progress is swift.