I am not sure what your feelings of guilt are about, but please take this advice:
"treat yourself the way you would treat your best friend if they were going through this"
this disease is the monster and the thief of our most beloved, there is nothing we can do except our best, but it's the disease that steals them from us both whilst they are alive and suffering and at the end when they are taken completely
As hard as it sounds, I think that a faster road is better than too long of suffering.
Find all your best memories of your dad and store them up together and parade them to yourself as a wonderful thing that you still have and that no one can take from you.