When to quit?

Hi. I've been an ICU nurse at a level one trauma center for a little over a year. Prior to that worked at a step down unit for a year and a nursing home before that for a year. It's been rough; some of the trauma nurse and the trauma MDs seem very critical of me, in particular, and I've made mistakes, including med mistakes, but work really hard and try to be super conscientious, pay attention, and ask lots of questions. I made a med error recently were I gave a med that had been dc'd but the order hadn't been taken off by the previous shift. I guess I don't know when to say I suck at this and should give up. One of the trauma doctors seems especially "fed-up" with me.