Wow, TooShort. I've sorry to hear the tough time you and your family must be going through.
If you think about it, Christmas is all about the hope of eternal life. I don't know where you stand on these things but I agree with the great thoughts expressed here. I'm sure the last thing your grandfather would want would be for you to have a terrible Christmas. I think it is great thing to celebrate the life of a loved one lost at Christmas. In my house, we remember all those we have lost and think of them as celebrating with us on the other side - much as the angels mingled with the shepherds on the night Christ was born!
Thoughts and prayers are with you from here, my friend.