I was my father's caregiver -- and I still ask myself that question. I think that it is rare for anyone to feel that "they did enough" after someone we love dies.
I think the mind wraps around these questions in a vain attempt to make death something that we control. It is almost like we would rather blame things we did or didn't do than accept that we are pretty powerless when it comes to dying.
My "unhelpful" thoughts (that is what I label any and all thoughts of "why didn't I", "if only", and "I should have") are usually best handled with some well-thought reasoning. I ask myself if I knew my father was going to die that night. Of course, the answer to that is NO. Then I ask myself if I did know, would I have been willing to do anything that night for him? Of course, the answer is YES. SO, since I couldn't have known he was going to die that night should I feel guilty that I didn't do anything? No. Perhaps some self-talk like this could help you, too?
I am sure your father knew that you love him.
I hope you can find peace in that knowlege, too.