The truth is, we don't realize how strong we are. When we have no choice, we do the impossible as well, because that is what is needed.
Once I heard a psychologist speaking in the radio and she said in case of long illnesses, grief starts when the illness is discovered. I can tell you, this is so true. In 1.5 years how much my mom had, I went through a lot of phases, from denial to acceptance, but I knew one thing, that I have to give her everything possible, and I did. I have my conscience clear, no regrets, but I still ask why did she have to suffer like that?
Anyway, I can understand how you feel regarding your grandfather and am glad you get to continue his work and take care of his farm. It means a lot to him, trust me. I'm sorry you're expecting bad news about your dad, but unfortunately this is the course of life.
When he died I felt lost for a little while, but I also felt relief. He was in a lot of pain.
I know exactly what you mean and only those can understand this who went through it.