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My name is***** am happy to answer you question.
Considering your cats age it is a very real possibility she may have one of the more common older cat diseases- hyperthyroid disease, diabetes, kidney failure, inflammatory bowel disease, cancer. Based on what you have described I would be very very suspicious of kidney failure. I recommend going to see your vet for a good check-up. They are likely going to want to do blood work and urine analysis to rule in or out the above different diseases.
Depending on the disease diagnosed there is the possibility they may be able to get your cat back on her feet but it is really hard to say. I am sad to say that there is the possibility that despite best efforts her quality of life may never improve beyond how she currently is.