LOL That is exactly how I felt when I started playing The Sims again for the first time in decades. Not the same thing, obviously, but I had to look up all the cheat codes I used to know by heart and how to enter them. It does come back to you, though. It takes a while for something to become a habit, so stick with it! It'll get easier in time!
I'm glad you were able to get two chapters done so quickly, and I saw that you already have comments and kudos, so you're off to a great start! AO3 is a HUGE site. I'm sure it varies from fandom to fandom, but they seem to place more emphasis on kudos than comments. I get far fewer comments there than I used to here, and many of the people who have commented on my stuff are friends who followed me there from here. (Thanks, Steph! LOL) Yet, if you sort by comments, the stories I've written since I started posting on AO3 are among the most commented BSB fics, so I can't complain. It just seems like there are a lot more silent readers there than there were here because we always made such a big deal out of leaving reviews. I still miss it here!