...But you need to invest in a saddle that aids you - because this one is hindering you. And I believe, that no matter how hard to try to correct your chair seat, you'll only get so far because of how this saddle naturally places you...
I have several saddles - Western, Australian and English. Not surprisingly, they all fit a bit different. Also, my youngest daughter uses the western one, which is too large for her but I haven't found any smaller at a price I'm willing to pay that would fit my horses - and she doesn't mind.
I've pretty much concluded that it is best to ride the saddle...put you butt in the deepest part, adjust your leg so the stirrup strap is vertical, and accept what that gives you. If it is a chair seat, so be it.
My belief is that if the stirrup leathers aren't vertical, you can't have a steady leg, and the deepest part of the saddle is where gravity puts your butt. So you can fight your saddle and have your leg swinging around putting you off balance, or you can accept what the saddle gives you and know that you just won't be as good as you would in a better fitting saddle.
Does that make sense?
From a jump perspective - as a reader, not a rider - would that mean the rider would need to lean more forward than usually desired in a saddle that forces a chair seat, so she could keep her center of gravity over the stirrups?
Remember - I'm asking, NOT advising!