sorry to hear about your dad
The closest thing i had to deal with was my grandmother having parkinson's and watching her go from an independent able person to being stuck in a wheel chair and not even being able to talk and eventually dying all in the span of about 4 years.
I just always tried to remember how she was before and think about the good times. It was kind of a relief when she died since we knew she was no longer suffering.
It was really hard since i grew up around her. She lived about 15-20min from me my entire life so she was a part of my life, not some relative i saw on special occasions.