Aren't you afraid someone will steal your work?

I want to post a fanfiction I'm writing. But I reaaalllly love it, and I'm also afraid that I should tried to publish it instead (it's not canon), or that someone might steal or copy it if I post it on Ao3. What do you guys do against that? Especially since I've seen some works being stolen, and also some works that could definitely have been published for real being posted on the platform. Also, do you all post before finishing the story? Because what if you post a few chapters and then people demand the rest of it and you need a few more months? I'm not good at writing regularly.

Also, I need a beta reader, but once again, the fear of having the damn story stolen stops me from searching. I know I'm ridiculously acting like it's some kind of baby I have to protect but it kinda is ya know?