There comes a time when you just have to tell yourself it's done and post it. There's no magic to it. You gotta do it, is all. And stop tweaking! Once I consider a story finished, I try not to read it until AFTER I post it. I tend to read my fic after I post to get a feel for how it looks on that particular archive. Then if I see a typo, I'll fix it. I don't edit that story ever again. I move on. If you constantly go back to a posted fic, I think you get stuck. But that's just me. :)

You'll be fine!