I was promoted to manager and I have no idea what I am doing

Good. That means you are paying attention.

You were promoted because you were excellent at the work. Then you were handed a completely different job with the same title and no training: managing people, a boss, and peers, all at once, with every mistake now public. The skills that got you here are not the skills the new job needs, and nobody said that out loud.

Here is the part that helps. Leadership under pressure is not a personality you have or lack. It is a set of habits, and habits are trainable, but only if you can see them first. And you cannot see them by reading. You answer books and quizzes as the leader you wish you were. You only meet the real one under a clock, when there is no time to perform.