I feel your pain. I recently went back to work after 7 years off and I had worked NICU previously for 8 years. Talk about nervous as a long-tailed cat in a room full of rocking chairs. I felt like the biggest fraud on the block, and still do many shifts. I certainly feel like more is expected of me since I did NICU before. I still feel like an idiot half the time and I've been back at work for almost a year and a half.
The nurse who emailed the manager was totally inappropriate. It's a shame that she was not more supportive and helpful. I've always heard that nurses have a tendency to eat their young; she's just living up to that. Hopefully, she's the exception and not the rule in your unit. Don't let one person muck up your whole mindset.
You will become more confident and comfortable as time goes on. There's nothing you can do except get the experience and learn from your peers. It does get better.
My biggest piece of advice for you would be don't EVER be afraid to ask a question. Whether it's respiratory, meds, asking the docs or seeking answers from your fellow nurses. It's better to ask and feel like a complete boob than to assume you know the answer or can figure it out yourself. Especially when you're starting out, you don't know what you don't know.