.....almost sounds like a stroke or something similar ! ...i am a nurse, so just thought it could be mentioned to your vet to give her a brain scan, just to check she is ok there.....this could possibly be remedied by treatment.
...it could be a reaction to something like food/meds, or it sounds also like an injury somewhere, (leg, nerve damage, spinal cord or internally) as she is obviously in pain.....she needs to be seen by someone determined to look for the cause, and quickly, as she and you are suffering, and to have this going on this long is being let down by your vet at the moment.
....i pray this isn't going to be expensive, and that something can be done to give her a good quality of life.
.....she is very lucky to have you as such a caring owner, good luck and my thoughts and best wishes are with you.
Debra x x x x