if I saw that situation in the form of a written exam question, I wouldn't think it was very complicated or difficult.

This seems like a symptom of discrepancy between your belief (I know the right thing to do in this situation) and your alief (I am not qualified/experienced enough to know the right thing to do in this situation). Sort of like walking on a narrow ledge 3 feet off the ground vs walking on a narrow ledge 300 feet off the ground. I wonder if there are exercises to work explicitly on aligning one's alief with one's belief. Maybe jimmy can chime in.

I do know that I feel very uncomfortable applying my own judgement to real-life situations.

I think that this is, really, the central point. From what you've said here, you know a lot about nursing. If given an exam question detailing just about any set of circumstances, you'll probably be able to answer it correctly (given that, sometimes, the correct answer is 'call the doctor').

But now, let's look at consequences. If you answer an exam question wrong, writing down a course of action that's wrong, then the worst that will happen is a few stern words from a lecturer, and maybe a low mark. Nothing serious.

If you get something wrong in dealing with a patient, then there are serious consequences; and those consequences are serious to the patient, as opposed to yourself. Like a good nurse, you are extremely averse to having a patient take on serious consequences. So in real life, you don't just want the right answer; you want the right answer and a high level of certainty that it is, in fact, the right answer. What you're approaching your supervisor for isn't, it seems, the right answer - you have that already. What you are approaching your supervisor for is the certainty that your answer is actually correct.

And he won't give that to you, because he doesn't want to spend the rest of his life rubber-stamping your decisions; he wants you to develop the skill of knowing how certain you are that your course of action is correct, and he also wants you to develop a reasonable threshold of certainty, such that in all but the most difficult of situations you can continue without his help.

That's how I see the situation.

Cool, it's great to hear that my assessment wasn't way off. :)

I agree that answering questions on an exam is a different skill from applying the knowledge in the field, almost in the same way that knowing the right thing is different from actually doing it. This is true even in software, where I generally have plenty of time to think about the right course of action. I can only imagine that it's way harder with the pressures of the ICU.