I'm working on a character who, throughout the course of the story, develops mild to moderate anxiety. I don't have anxiety myself, and while I could look things up, I think he'll be more realistic if I get details from someone who actually has anxiety. If you have anxiety, please give me tips on how to write this character!
I'm no expert either, but I've struggled with moderate to low-level anxiety for most of my life. I can't even remember when it didn't affect me, actually, although my parents tell me I used to be very brave, outgoing, and adventurous. But somehow, at some point, a switch flipped and I got less confident, more withdrawn, and more prone to feel anxious about even very small things.
My point is, I'm not sure how one might "develop" anxiety, and I'm fairly certain it's a slow process. There are probably genetic factors involved too, so that might be something you want to research pretty deeply.
As for what anxiety is like, I actually have a thread over in Conditions where I answer people's questions about it » https://www.notebook.ai/forum/conditions/i-have-some-experience-with-anxiety-and-coping-with-anxiety-and-am-willing-to-answer-questions Again, this is only what my experience with anxiety has been like, so don't rely on it too heavily, but I hope it can be at least a little helpful! ^^