Is it possible that the saddle is just too heavy? I have a gorgeous western show saddle that was given to me as a gift years ago before I owned my own horse that I never ride in. When I bought my first horse, a 14.1 hand MFT/Morgan Heinz 57, I bought a cordura saddle for daily use to save the leather show saddle. The saddle fit him fine, but weighs almost as much as I do. Before I would show I would ride in the heavier saddle to get him accustomed to the weight, so this wasn't a sudden changeover problem. As the summer wore on, whenever I rode in the show saddle, he carried his head higher and higher and was less and less responsive. Hard to move off, and hard to stop. I would pull the leather saddle and put the cordura back on, problems would vanish. If all you're doing is trail riding, a nylon or other synthetic will be fine. I now show English, my sis rides the saddle on her chunky QH, who is comfy as heck
If you can, find someone to lend you a synthetic to test ride and see how your horse reacts. If that works, eBay
the leather and recoup some $'s.