I suppose I'm lucky, I've only had one grandparent die so far and that was my maternal grandmother who died of brain cancer when I was 5 or so.
Her husband, my grandfather, isn't looking to great, though. The last time I saw him it suddenly occured to me that he was old and frail and it hit me like a ton of bricks knowing that in the near future he won't be here anymore.
In summery: I can only imagine what you're going through and offer my sympathies.