How do I learn the unwritten rules of the professional world?

You made it into the room, which is a real achievement no one can take from you. Now you keep hitting rules and codes no one taught you, the unwritten kind that others seem to have absorbed at home or in schools you did not attend. That is not a gap in your ability. It is a gap in inside knowledge, and inside knowledge can be learned. These unwritten rules are just a culture, and every culture can be studied from the outside by a sharp observer. Watch how the people who move easily actually behave: how they email, how they disagree, how they ask for things, when they speak and when they wait. You are not less than them. You simply started without the map they were handed, so you draw your own by watching closely. Find one person a step or two ahead who seems generous, and ask them directly to explain the things that confuse you. Most people are glad to help when asked plainly. There is no shame in the question. The shame would be pretending you already know and stumbling in silence. You earned your place in the room. Now gather the map, and soon these rules will feel like yours too.