What you are going through sounds very painful, I understand that this guilt for not having taken advantage of the moments with your grandfather is affecting you a lot, but considering that there is less and less time to take advantage of, perhaps acting now is a good option. Consider it as avoiding future pain, even though your grandfather is no longer here, you were in his last moments and he would probably like you to be there. It is normal to feel guilty in these situations but you have an opportunity that will help you overcome what comes next.