I had a similar experience. Mine was a combined balance and flap issue. Each CWD that I had seemed to have a balance further back in the seat. Combined with my rep putting me in a forward flap Mademoiselle rather than the straight flap that I need. It was impossible to keep my foot under me. My trainer would tell me to sit further forward in the saddle and I simply couldn’t.
I switched to a Voltaire Palm Beach and I’m not kidding when I say it changed my life. People giggle when I say that, but it’s a revelation when you don’t have to fight your tack any more. The balance is perfect for me, the flap is the right degree of forward. It puts my leg in a perfect and secure spot; you shouldn’t have to work to keep your leg from being in the wrong spot. All of this sits me up more in my upper body, opens my shoulders, and softens my elbows.
OP, I’m not entirely sure the saddle is too small, maybe a half an inch or so. But the balance looks to put you too far back. Possibly the stirrup bar too far forward. But they both exacerbate each other. I realize the saddle fits your horse, but that doesn’t much matter if it doesn’t fit YOU too. See if you can try some other saddles that will put your butt further forward and leg further back; this will automatically sit you up taller. Bend your elbows and that’ll keep your arms from pulling you in to a folded chair seat.