Hi!
First of all, congrats for getting an AC in the first place; that in itself is already an amazing achievement.
Honestly, I think I know what you mean; you know you can’t change anything about what’s done, but for some reason, your brain keeps on replaying previous moments/ the worst parts, and you keep on thinking of ways you could’ve done better. I think most of us have been through this at some point.
I’m no expert, but here are some of the things that I did to help myself through that stage:
1) write down the ways that you feel you could’ve improved e.g., how to better structure answers etc. After all, this is your first AC and I’m sure there are many more to come- you can take your personal criticisms and turn them into practical future improvements.
2) once you’ve done that, try not to think about it any further. This is the hardest part but I feel like once I’ve written down all of the reasonable improvements I can make, I’m able to control myself better and not fall into a hole of self-deprecation and doubt
3) for worst case scenarios, just forget that firm and that interview ever happened and move on lol. I did this with a firm once (kept busy with work, uni, other applications etc) and was confused for a moment when I got a PFO from the firm because I managed to blank my experience with them from my memory
Note that these are entirely personal and might not be objectively the best methods to take, but I hope they help! Would love to see what everyone else has to say about this too