You are correct about the symptoms of PSSM and it doesn’t sound like your mare is showing those. Some other muscle problems can be related to Vitamin E or Selenium deficiency, but your description sounds like she really just needs more training. One other thing that I can think of is maybe consider a chiropractic adjustment. Some horses with back issues are reluctant to canter and may be girthy and it doesn’t necessarily have anything to do with your saddle fit. Hock issues may also contribute to a horse not wanting to canter. If you start riding more consistently and don’t see improvement then I would recommend you consult your veterinarian to rule out health issues. Good luck!