I've been in ICU about 5 years. I'm still anxious about messing up and killing someone. I'm less anxious about it now that I have some experience under my belt, but you're at a difficult level of experience right now. You're getting more competent, which means you can appreciate how many ways in which a simple error could cause irreparable harm. You likely used to only understand about half of these ways. Sometimes ignorance really can be blissful.
I find it helpful to remind myself that our unit has a very good spirit of teamwork, and any time your patient becomes more critical and starts alarming to high heaven, bleeding profusely, having life-threatening arrhythmias, etc....you wind up with at least a couple of colleagues coming to see what's going on. And if you've missed something, they're pretty good at asking appropriate questions and rapidly determining what you've overlooked or might have messed up, and then helping you fix it.
Also, even though it's not very kind to admit to, I like to remind myself of things my colleagues have messed up before, which often horrify me when I hear about. Because if my colleague bolusing an entire bag of insulin or letting their chest tubes clot off didn't kill the patient....well, patients can be hard to kill sometimes. Even when they're very ill. Your body really WANTS to stay alive. It's full of compensatory mechanisms that allow people to have a heart rate of 30, or a SBP of 60 or a sat of 57% for awhile before they actually croak. That said, sometimes a swift breeze could take your patient out.
Finally, if you quit and your colleagues are more short-staffed, mistakes will be made and more people will ultimately have poor outcomes. You WILL ultimately make mistakes, whether you're made aware of them or not. We're all human. Just try not to make the same mistakes multiple times.
1. Mistakes are human, patients would die without the ICU, you're providing a service with a net positive outcome, even with occasional mistakes.
2. Humans are hard to kill (mostly)
3. A little anxiety is helpful, a lot of anxiety is crippling and deserves therapy and/or meds.
4. An adverse event is very rarely exclusively YOUR fault- there are many members of the healthcare team, as well as the nursing team.