Dear Jackie,
I understand how you feel. It takes a long time to get over the guilt. I also felt i should have done more. But, the truth is that all we can do is our best. I know you did your best for your Dad and he knows to. It is NOT your fault. You did not cause his cancer and you did not cause his death. Cancer did. Blame that beast. Don't you dare take any of the blame.
As far as the pain. It is horrible for awhile. Let yourself grieve. Be patient with yourself and understand that someday your grief will subside. But first step is accepted the fact that you did nothing wrong. I will keep you in my thoughts.