My god that is horrible. I am so sorry. I am sure that your dad would not want you to torture yourself so.
I had something similar happen with my dad. To this day I wish we would of just brought him home to die but we were thinking maybe he would get better. Maybe we would have a little more time.

Its been over 20 years now and I still think about it but I do know he loved me and would not want me to keep hurting myself over that decision.
Please try and let it go. We do the best we can. If your decision was not done out of any kind of malice, which I am sure it was not, what else can we expect. We have no crystal ball. We cant go back and have a redo. You are just human doing the best we can in this life.