Let go of the crutch and try doing what your coach tells you. It honestly boggles my mind that you pay this person to instruct you each week but don’t try to achieve the standard they have set out for you. What a waste of money.
I know you probably mean well… but I actually find this comment a bit offensive. If I was using my short arms as a “crutch” I would not be on here trying to figure out a way to work it out. In all honesty - if I can barely even touch the pommel of my saddle with the tips of my fingers when sitting in it how is it even reasonable to assume I can ride with my hands below the withers???
The reason to posting about this was to hear opinions on what it the most effective of my three “less than perfect” options for riding with my particular conformation: A) hands very low but with striaght arms and leaning forward, B) posture/elbows/arm correct but with hands too high, or C) somewhere in the middle. So far I am going with the last option.
And although I like my coach and I feel very lucky to have her (I live in an isolated area so having access to someone of her calibre is a blessing) - I do not think every coach knows everything and is always “right” - I do not turn my brain off when I turn over my money for lessons. 
As an update… after a rather awkward lesson where my coach spent a lot of time with her head tilted watching me with a weird look on her face - she has laid off on the hands below the withers requests - I think she actually finally saw what was happening. :winkgrin: Our lessons have been going much better and my back and shoulders are far less painful after riding now and my mare is excepting the contact much more consistantly.
As for it being my saddle - perhaps maybe a little bit - I am currently between dressage saddles - I am riding in my CC - but my new Jaguar dressage saddle should be here any day now - fingers crossed that it helps. And although my young mare is bigger than I prefer (I’m 5’2" and she is 16.1hh) she is no giant so I can’t see that being a major issue. Although her mother who is 15.2hh does fit me much better. Unfortunately Ysabel did not stop growing at the height I wanted. 
LOL - I started Yoga a few weeks ago too and my short arms have also been a bit of an issue with some of the movements, but the instructor just laughs at me. My husband says I am shaped like a T-Rex dinosaur with short thick legs, long body, giant head and tiny arms - LOL - but he still thinks I am beautiful. :lol:
