It sounds to me like your saddle is pinching, and at this point it's also likely caused some chiropractic issues. I do equine evaluations and clinics that include saddle fit through Welcome to Equi-Eval | Full Service Equine Evaluations
and would be happy to help if you're interested.
In the short term what I see is a saddle that's too wide - so it's rocking - and also pinching shoulders. This will cause tenseness in the neck, as well as a stiffness about 3" behind where the saddle hits her back from the motion of the saddle rocking off her back, then hitting back down. Your horse will likely compensate by taking shorter, stiffer (more upright) strides due to not wanting to extend through the shoulder and neck, and the hind will lack flexion in the hocks and stifles even though the issue is actually in the mid-lower back.
Circles will become increasingly hard where the horse has to bend at all, and leaning will become more of a habit to protect from pain. Because the saddle is pinching as well as hitting in the back, the horse will eventually develop an unwillingness to move forward that will likely start with a simple resistance, slight hesitation, or mild resistance to the aids, but eventually will likely progress to refusals to go forward, not wanting to stand to be mounted, or not even want to stand to be saddled/girthed at all. If you jump - this can also manifest into bucking (and bucking usually ensues even with the best of horses on the flat over enough time with an ill fitting saddle).
Please keep in mind you can NOT pad a saddle to make it wider, so a saddle that is too narrow will ALWAYS be too narrow and no amount of padding will help. Think if you wear shoes that are too small and they cause your heels to blister, adding pads to cushion your heels will NOT make your shoes fit better. In fact, it's more than likely going to have the opposite effect causing the shoes to fit you even more tightly - and more uncomfortably. How does this translate to how YOU will move, walk, and act? Then translate that back to your horse.
Also please keep in mind this is a lot of general information (above) and as I've not seen the full pictures of your horse to do an actual evaluation, I'm making some assumptions. If you're interested in learning more, please feel free to check out the site and email me and I'm happy to help/do a full eval for you :)
If you do need another saddle - I HIGHLY recommend the Bates and Wintec over any other line with the changeable gullets.