It will be two years since my dad died this April 22nd. He also had a slow decline and became very frail and weak for a few years before his death. I'm so grateful for the times he comes to me in dreams and I can actually hear his voice again! Some days I'm able to be more objective, and like you I'm glad our loved one's pain is over. But some days, I just want to talk with him again. No matter what your reality of your relationship with your dad (I also own a pair of rose-colored glasses), you're also grieving the passing of all your perceptions of what a father is -- in my case, some one who always took care of me physically. When he was so ill, and since he has died, I've had to come to grips with several fears surrounding my own eventual leaving, how I've lived my life so far and changes I want to make so I don't "waste" my future, and the truth that in many ways I'm "alone."
Be gentle with yourself