Have you considered that maybe it isn’t the saddle making her sore? When her butt comes up that’s going to change her balance and how she’s using her body, and maybe she’s overusing those back muscles as she finds her balance. Just a thought.
When I used to start young horses, I would often just give them a few weeks off when they came butt high so they could figure out where to put all their limbs again before having to deal with balancing a rider.
I was a big fan of Hennig, but you can’t get them anymore. After spending thousands trying saddles on my 3 and 5 year old horses, I ended up with the Fairfax World Class (two of them!). Big fan!!! I can ride in it because it has a narrow twist, and it is wool flocked, yet it has adjustibility for a growing horse. I love the should blocks, which my narrow horse can handle, but my wide guy hates. It is great to be able to experiment with them. The World Class is not cheap, between 5-6k, but I expect it to last at least until they stop growing (age 8-9) and maybe beyond. I had to buy like three saddles for my past young horses during this time. I like that the panels are lined in prolite (like the Hennig panels were lined in memory foam), but wool flocked for adjustment. The gullet plates and shoulder blocks are adjustable. I prefer not to ride with a saddle fitter pad. It takes me too far away from the horse, so this was a great solution.
That could be affecting things too but the saddle doesn’t fit. She’s been very reactive under saddle the last few days and a little standoffish lately. I think I’m going to end up treating her for ulcers again before going forward. She is so sensitive and I haven’t been able to break the cycle with potential stressors yet.
Late to the party here, but I have a broad horse with a rather short back (or I should say short bearing surface–Arab/WB X) and he’s going really nicely in the Bliss Loxley warmblood tree. Another short-backed horse in the barn has just got the same saddle but with the regular tree. They are moderately priced, well built and very comfortable.
Also for the short wide backed types, I’d recommend reaching out to Adrienne Hendricks. I really like the saddles that she reps (Lovett & Rickets) and they were originally designed for arabians. She works on all sorts of saddles and has knowledge. She is currently doing a series on her Facebook where she is taking panels off of various saddles that came in and photographing the trees to show what happens to saddles that are adjusted on saddle presses (which seems to be a popular way of fitting saddles at the moment). (My Fairfax saddles have an adjustable tree, but it is adjustable by replacing the gullet plate and adjusting the flocking not by using a press.)