You have been given some excellent advice, I agree wholeheartedly with what has already been said. I also wanted to mention that it may not only be the western saddle that is the issue, is that your mare in your avatar? If so I would venture to say that the english saddle is a poor fit too judging from the way she is carrying herself.
It sucks that you are looking at replacing so much gear plus seeking diagnoses but at the end of the day the only thing that matters is that your horse is happy and healthy. Good luck!
I agree with this. She's really hollowing out her back to get away from the saddle if that is her in the picture. She also looks like she's trying to get away from the bit.
You said your hands are better now, but if you guys were having trouble before and a saddle fit poorly, she can be sore in a LOT of places. Her neck, her back, her back legs. There is a horse at my barn that is having this exact same problem. My trainer is working with her, she's also a trained masseuse and saddle fitter. Everyone said she was just being a witch, but she is in a LOT of pain. She is a good horse though, and lets you put on the saddle and get on her, but I wouldn't if I were her. The little amount of crap she is giving in proportion to how much pain she is in is astounding.
I wouldn't just have a chiropractor out there, try and get a masseuse if you can. Her back is probably crooked and inflamed from trying to get away from the saddle, and her muscles are probably tight and knotted. A chiropractor will fix her bones, but if her muscles are tight from the saddle, her back will just go out of place again. All of her muscles should be soft if you push into them. They should feel like the muscles on her chest when you push them, or by her flanks. All of her muscles should be that soft. On her back along her spine (not ON her spine, to either side of it), if you push along there, do the muscles feel tight? If they are, she is in pain. Rub along her spine (on either side of it), but push firmly. Does she have any muscle spasms from that? If she does, she is in pain. Get really close and look for even the smallest movement. Or if she really hollows away from you pushing, that isn't good either. Try and look at her back from a level perspective. Is one side higher than the other? It just takes a -little- bit being out of place to really put a horse in pain. Run your finger down the middle of her spine. Do you feel any big ridges? Her vertebrae might be inflamed. Are any off the the side a little bit? Does she crank her tail to one side when on the lunge or you're in the saddle?
Also, how are her feet? And I would have her teeth checked.
Does she pin her ears at a walk sometimes? The problems compound when working harder, so she may be giving you signs at a slower pace.
HOWEVER, if all of this checks out, she's not in any pain, her teeth and feet are good, she may have learned from the western saddle that if she throws a fit you won't make her work. But I would not even consider that until you have every other avenue thoroughly checked.
Don't sell her for behavioral issues. Unless they get incredibly dangerous, most can be fixed. :)