Join Date: Dec 2010
Location: southern Arizona
I'm pretty inexperienced, so take this with a grain of salt:
My daughter tends to post the way you do. In her case, she is riding a saddle that is too big for her - which is because she has to share a saddle with the adults in the household and insists on only riding western. Western saddles for small Arabians are hard to find, and the only one we've found so far is a 16" seat.
When you ride, gravity wants to place your feet so the stirrup straps hang straight down. That is the normal place for your feet to go, and anywhere else is a fight against gravity. Meanwhile, your rump seeks the lowest spot in the saddle - gravity again. If the saddle is too big, then gravity is trying to put your feet out in front of your rump - the 'chair seat'.
When riding with a long leg, a chair seat doesn't hurt you much. Your leg just angles a bit forward. However, when riding with shorter stirrups, your leg forms an upside down L. When you post, you naturally want to balance above the stirrup, and the only way that can happen is for your butt to come forward...and since the stirrups are not going to go down, that means your butt has to go up - way up.
The Dad in me refuses to buy a $1500 saddle for my growing daughter. It would be an annual expense. And since she prefers riding western, I told her to use a longer leg and just accept that she doesn't need to get her heel all the way under her while posting.
Unless your jumping higher fences, I'd recommend lowering your stirrup and accepting that you don't need to reach a rest point with your hip suspended above your heel. Strengthen your leg (squats?) and practice posting with the goal of barely getting above the saddle. Instead of thinking of it as getting up in the stirrup, think of it as stretching your stirrup down just enough to take the impact out of a sitting trot.
The purpose of posting is to reduce impact on the horse's back, not to get you in a balanced standing position.