I'm sorry your father is not doing well and you are going through this. I believe I know how you feel. I watched my father as he deteriorated until his death from cancer. I know this is difficult, but I would suggest you spend as much time with your dad as possible. I treasure the time I had with my father.
I was in the midst of a deep depression during this time, but I started seeing him once a week, then a few times a week and as his cancer progressed, everyday. He would ask me to sit with him as he took his nap and I did, holding his had. As I sat with him I would think about the great times we had through our lives. When he woke up I would share those memories with my dad.
The day before he died, as he was falling asleep, I told him it was OK to let go, that we would be OK. As I was driving to his place the following day, I got the call. As sad as I was I found solace in that he was no longer suffering.
Regarding the living will, it's a document that spells out your father's wishes regarding medical treatment if he becomes incapacitated and is unable to communicate. My father had one, I have one.