My confidence is so, so weird.
I grew up doing theater semi-professionally. I've been on stages acting, singing, and dancing in front of thousands of people. I always had the mindset of, "There are so many people here that I shouldn't care what they think. I don't know them, they don't know me, but I want them to at least remember the performance I give."
Writing for people online feels the same way to me. Before I go on stage (publish the chapter), I'm a little nervous. I practiced and practiced (wrote and edited) for this day, and I'm hoping that my performance (writing) is enough to sway people.
My confidence stems from me knowing that I've done things that should be unbelievably nerve-wracking, so whatever I'm doing should feel easier in comparison. When it comes to skills or talents, I have a thing or two that I know I'm good at, but I'm only confident in those things because I know that my opinion about those talents have been validated by enough people that I have no room to doubt it.