How do I get confident treating patients' breathing on my own?

Following the protocol is one thing; standing on your own and deciding how a patient should be breathing is another, and the fear you feel at that line is honest. It means you understand the stakes. Breathing is not something to be casual about, and a person who felt no fear here would worry me more than you do. Your fear is proof you take it seriously. Confidence in this does not arrive before the skill; it grows out of it, slowly, through supervised repetition until your judgment has been tested enough to trust itself. Right now the protocol is your scaffolding, and that is exactly what it is for. You are not meant to throw it away. You are meant to lean on it while your own assessment quietly becomes reliable underneath. Seek out the chance to make the call yourself with an experienced person beside you, and say your reasoning out loud before you act. Let them confirm or correct it. Each time your judgment is checked and holds, the fear loosens a little. Confidence is assembled from many small moments of being right with a net beneath you.