I have the same problem - although, for a long time, it manifested itself as leg forward/upper-body perching. Then I fixed both problems while I was a working student, and now that I'm home and riding alone, the upper body is fine, but my **** chair seat is back! It's MUCH worse when I'm riding in my dressage length.
What helped me when I was a working student was abusing myself, haha. Basically, my trainer put me through position bootcamp, and it was my responsibility to keep it up. What you don't want to hear, and what I will advise, is to take your stirrups off your saddle. Take 'em right off, and leave them off, for a month. Do everything you usually do - jump, flatwork, trail-riding - and you will be surprised how much your foundation changes. You'll become more stable, because you won't be relying on your stirrup. Ride as much as possible, and without stirrups on all! (Unless it's like, a barely trained baby. Use judgment
)
When you're comfortably situated, pull one leg up so your thigh is pointing down and your foot is up near your butt. Grab your foot and pull it your thigh. Then, without moving your thigh, let your calf fall back down and hang. Do it with your other leg. This will streeeetch your legs out and put them in a correct dressage position. At the trot, do a lap around the ring sitting. Then one posting. Then one in half-seat. Do "frog-legs" at the walk, trot, and, once you can figure out the timing, the canter. A frog-leg, if you've never done it, is a little tricky to explain - you pull your legs out to the side, so they're not touching the saddle, lift your knees, and push out your legs, like a swimming frog. That's all about timing.
Like I said - abuse. Your body will hate you for a few days. But it DOES work, and as Mercedes said, anything worthwhile needs to be fought for. Plus, at the end of the month when you're a total DQ (as my two CCI**** and Olympian trainers took to calling me) you can impress people with your fabulous stirrupless sitting trot. SO worth it.
