see my thread (sorry I don't remember the title) but started by arabiansrock, lots of kind people responded to me, I have exactly the same problem. there were several good ideas out there.
My solution was to order a Fairfax (leather version of thorowgood) but it won't even be here for another 4-5 weeks. I am currently very seriously contemplating buying a thorowgood t4 to use in the meantime as my current saddle cannot be put on her again, it hurts her too badly.
good luck and please let us know what ends up working for you.
My 15.2 hand guy was so short backed, I had a rider come up to me at a show and ask where I put the saddle . I used a Niedersuss Symphonie in a 17.5 with an anatomical girth and didn't have any problems.
it's not the edge of the earth, but you can see it from here
My stallion takes a 68" sheet... and has a scapular that goes on for days.
He has done well in treeless (Fhoenix), Reactor Panel (short panels for him, correct seat for me) and Passier.
What you DON'T want is flat gussetted panels, as generally they'll dig into the loin. Panels that sweep up behind will allow a bigger seat for the rider while not being too long for the horse. I manage to ride him in an 18.5" Passier, which actually is a bit big for me, (18 would be better) but he adores it, so, I don't argue.
I have a little TB mare that I'm having similar problems with. She's 16h, very petite, and much more "square" than rectangle.
After much trail and tribulation, I've sent my Schleese Wave back to the manufacturer to be 'shortened.' They plan to cut back the panels and cut out the bit of padding that extends past the back of the cantle.
I LOVE that saddle - just need it to fit my little mare!!
I have the new Albion for short-backed horses. It's called the Classical. My short-backed, forward girth groove, laid back scapula, and big moving mare loves it. She also tolerated the prestige D2000 and the KN. I noticed all of these saddles have short and small panels that are not flared in the back. The big flared panels that are so fashionable now--because they do look comfortable--were torture to my mare's loin and invariably pushed forward onto her shoulders even with an anatomic girth.