Friend, I had open heart surgery at 53 last year and almost died. I went through all the emotional and logical changes that you go through when you truly believe that you are about to check out. I can't help you stop hurting, but I can give you some idea of how your father felt at the end.
Let me assure you that if your father lived ninety years and he spent his last conscious moment in the arms of his wife, with his children having grown into responsible adults (the way he taught you to be), and no regrets that he was leaving anything unfinished (he was, after all, 90), then even though the unknown might have made him a little bit uneasy, he certainly left us feeling that his long mission in this life had been completed, and that he was going to his well-deserved rest.
It was time for him to go. He had accomplished his mission. God needed his soul for another purpose, which we are promised we will know some day. Where he is now, he's not old, or frail, or sick, or weak. He's the way you remember him. He doesn't need to depend on anyone. And if he wants to, he can watch Grange and Thorpe and Gipp and all the other great ones play, in person, and maybe even throw a few balls with them.
I lost my dad over twenty years ago. Even today, if I listen very hard, I can still hear him. As time heals you, you will begin to hear your father again.
My condolences.