Stirrup length, the posting trot, and chair seat

Today let's talk ALL the things that drive me crazy on a daily dressage riding basis, things that confound me no matter how hard I try to get my head body parts around them. They all tie together in a neat little bundle of equitation catastrophe.

behold! the beauty of none of the body parts working in unison

My biggest problem--well, the biggest problem in in this specific subcategory of all my many problems--is with posting the trot in my dressage saddle. I work tirelessly to try to correct this enigma. I park Bobby in front of the mirror and try to manipulate my body into the position it needs to be in to post upwards and forwards with a long, straight leg and long, straight back. I'm not above grabbing mane or the front of my saddle, but the thing I fuck with the most is my stirrup length.

I don't keep my stirrups the same length for more than a few rides in a row. One day they'll be fine, and the next I'll get on and they feel like they're three holes off. Some days short feels too short, long feels too long, or the right will feel like it belongs to a different rider than the one on left. 

This applies to my jump saddle as well, so it might be something like leg/hip tension that's making me tighten my muscles and not stretch as well as the ride before. Also perhaps stirrup adjusting barn trolls that invade the tack room overnight? Let's not rule out all the possibilities. 

On the days where my stirrups feel too long, I feel like I don't have a solid base to post from. "Don't post from your stirrups!" is obviously a good answer to that, but it's more that I feel like I'm reaching just to make contact with my irons. When they're too short, I feel like I'm blasting ten feet out of the saddle. 

So the problem starts there. My leg is all hibbily bibbily having its own personal identity crisis, and the rest of my body is just like abort!!! My core collapses, my shoulders slouch, and I can barely get my ass out of the saddle without a great effort of throwing my fat roll forwards and following it in a vague interpretation of the fetal position. Enter the chair seat.

collapse all the things

My saddle has very tiny knee rolls which I generally prefer as I have obscenely long femurs and knee rolls tend to yank my leg around into not a comfortable position. But I have to wonder if having more help up there wouldn't force my leg to behave itself and stay in that ideal hip to heel alignment, and if in turn my stirrups wouldn't constantly feel like they weren't at the wrong length. 

Is any of this making sense? Honestly I'm not sure how much sense it makes to me either.

WHICH IS PROBABLY WHY I CAN'T POST CORRECTLY.

Actually, though, some days it's just fine and I have zero issues making my body behave itself.

WHICH JUST MAKES THE PROBLEM WORSE.

i would settle for this as defualt

Does anyone else have this problem? Where your stirrup length never feels right? What is the right length? Any words of advice on how to keep my leg underneath me better in a dressage saddle? This isn't a jump saddle problem, by the way. It's exclusive to dressage.

WHICH PROBABLY MEANS I NEED A NEW SADDLE.

WHICH MEANS I QUIT RIDING.

possibly if i shove my ass forward ten feet it would help?