The problem with people dying is that there is no way to resolve complicate emotions with them anymore. Things you wish you could say have no place to be said.
Have you considered writing him a letter and putting it on his grave? It can be very cathartic.
That being said, my dad is still alive. He's in jail right now, and I'm not allowed contact with him because I'm a state witness, and if he doesn't get a really good plea deal, he may very well spend the rest of his life in prison, but he's still alive. In some ways, it's like he's dead. I still can't talk to him. I write to him nearly daily. I'm only allowed to send him postcards, and I believe it's an error on the jail's part that they let my mail through, so I can only talk about boring mundane things.
I'd put money on it that at some level, Kialua, you want the same thing I want: to speak to your dad and tell him how you feel and what you wished would have happened. And that possibility has been removed from you, just as it has from me (different reasons, same result).
Perhaps you could write him one short note each night and see if that helps. I know people who have done that, then printed all the letters out, ripped them up and tossed them into the sea to watch them float away. They've said it made them feel like they finally got closure. Maybe it would work for you.