Not good enough

I have been a nurse for three years now, for the past two years I have been on a surgical transplant unit but we do still take care of other medical and surgical patients. Majority of my time in bedside nursing I have loved my job, i feel like I have definitely found my passion and I feel so good at the end of the day having helped someone out. There have definitely been days where I hate my job: feel overworked, underpaid, under appreciated etc but it's not for long but lately I have felt stressed out and frustrated. My floor has been very helpful and I feel like the management team is usually there for the nurses and tries to help us out but The past few weeks have been hard at work with difficult doctors/patients and families, I started to become super unhappy and to top it off I feel like after 3 years of working i feel like I am not smart enough when it comes to situations becoming more stressful. I know that the things I go through will help me be a better nurse but I honestly feel so discouraged after having days where I know I could have done better. I hate the concept of "learning from your mistakes" cause I don't want one day that my mistake harmed a patient. Days like this make me want to get out of bedside nursing, Im scared that people's lives are in my hands but I have no idea what else I could be good at. Anybody else feel like this or have any advice?