The answer is already in the story:
"I'm trying to think if there's anything I should be doing right now,"
Naturally Harry thinks of what he could have done differently and/or what he can do better in the future, but his main conscious focus is "here and now". No past, no future, no daydreaming. Here and now. I think this is an excellent advice.
"I'm trying to think if there's anything I should be doing right now," said Harry Potter. "It's hard, though. My mind keeps on imagining ways the past could have gone differently if I'd thought faster, and I can't rule out that there might be a key insight in there somewhere."
I'd misremembered this-- I thought he'd been trying to get his mind off his possible mistakes, but couldn't, and I get the impression that the people in this part of the discussion didn't think he'd even been trying to get his mind off possible mistakes.
Actually...
This is a new thread to discuss Eliezer Yudkowsky’s Harry Potter and the Methods of Rationality and anything related to it. This thread is intended for discussing chapter 90. The previous thread has passed 750 comments.
There is now a site dedicated to the story at hpmor.com, which is now the place to go to find the authors notes and all sorts of other goodies. AdeleneDawner has kept an archive of Author’s Notes. (This goes up to the notes for chapter 76, and is now not updating. The authors notes from chapter 77 onwards are on hpmor.com.)
The first 5 discussion threads are on the main page under the harry_potter tag. Threads 6 and on (including this one) are in the discussion section using its separate tag system. Also: 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17,18,19.
Spoiler Warning: this thread is full of spoilers. With few exceptions, spoilers for MOR and canon are fair game to post, without warning or rot13. More specifically: