It's hard to see parents decline...and not have much support to call on.
When my mother was in the beginning stages of Alzheimer's, needed someone to look after her, and I had been appointed her conservator, she became very disagreeable. I knew that this was partly the disease, and partly just her personality. I put up with it as long as I could, until the day she screamed at me that she didn't want me to be her conservator anymore. I told her that the court said that she had to have one, that I was the only one in the family who was willing to do it, and had volunteered. Since I volunteered, I could quit at anytime, she would become a ward of the state, and live in a white tile room like "One Flew Over The Cuckoo's Nest". She remembered that movie, looked horrified, and immediately modified her behavior.
As the disease progressed, she became a perfectly lovely person to be around.