Was a certified Great Guy (TM). Captured a bunch of enemy soldiers in the war by himself, retired from the service afterward. He was an impossibly handsome and dapper gentleman who wore a suit everywhere and treated women with utter respect. He knew when it was appropriate to spout philosophy and when it was better to tell a blue joke. I just idolized him to pieces, he was a wonderful role model.
The cancer ate him slow and he became a twisted mass of pain in a bed. The second last thing he ever said to me was "growin' old ain't for sissies." The last thing he whispered was "love." It was all he could get out.
I have felt what you are feeling. I am sorry, when one hurts, we all hurt. I can say that I often meditate on the 'commonality of suffering', and it has helped me deal with these sorrowful thoughts you are describing. Perhaps you might consider trying it. Google the term and you will find it pretty easily.
Love and good wishes to you and your family.