My father had Parkinson's and in my experience the lack of agency he had when trying to do daily tasks even other than walking was a real mind-killer. He was dependent on my mom (and later caretakers and skilled nursing facility) to do anything like feeding, toileting etc. Obviously when he was later bedridden mostly due to the inability to move, it probably drove him even more crazy.

He decided early on to never get this treatment and I feel sad to this day that he couldn't have lived a happier last few years.