I lost my grandfather a couple of years ago, was the most painful experience of my life. (I'm the oldest grandchild, and grandpa was pretty much my role model and hero all my life.) Anyhow, his health had been going down hill for a while, he wasn't nearly as sharp as he once was, and his memory was totally shot, and we all knew it was coming any day. We thought we were prepared, but when the day came, the reality of the situation hit everybody like a freight train running over an ant colony. Every day since then has been slightly less horrible, but I sure as hell miss the old man more than I ever thought possible. I can't imagine the torment my grandmother has dealt with over the last few years! The world is a darker place.
Anyhow, I hope you make the best of what time your dad has left. Be there for your mom, and be strong, especially when you feel weak.