Perhaps you can hold on to hope that the diagnosis will become clear, all the while being sure her doctors are treating the symptoms, including the mood/behavior issues. And I hope you are taking care of yourself - caring for aging/ill parents can be a long, exhausting road.

My father-in-law died from dementia (I realize this is not what your mother has) and it was a harrowing year for my husband and me. We, too, watched an intelligent, funny man quickly change into someone we barely recognized. I had to keep gently reminding my husband not to expect rational behavior from a person who was not and never again would be able to be rational. It was easier for me to see that since it he was not my father. Ask for help if you need it ... most people love to be helpful at times like this.