My condolences Mike. I truly understand your feelings, as I went through the same thing with my Dad. Being there is a blessing even though some of the thoughts you may have feel betraying now. I went through those feelings also. It was so bad for me near the end that I had very negative thoughts when next to him, unconscious, in the middle of the night, alone. When the end came it was a relief. I'm sure you know this and it is said frequently, but time does help dismiss the negative emotions and you will be left with the good memories. The awful things you've gone though being there for him will fade. Nothing ever replaces the loss, though.
Be proud you gave him the gifts you did.