I've had occasional horrifying moments of cognitive dissonance where I realized a character looked like X at first (hated them!) and then, guess what, they're not even a frenenmy of your hero, they're just a reality check, and they won't shut up and get run over so easily as all that. And...then they get interesting. And maybe it turns out they're a lot smarter than your hero, in lots of odd ways.
To make a fairly sloppy HP fandom analogy (because I'm not expert on it and don't write it myself) this might be like finding out Snape (nasty teacher who you hated at first) is your best bet among all the teachers to keep things running at the darn Hogwarts school, and damn good at your back when the whole world is turning into a combat zone. And then wit and intellect is always hawt, isn't it? Arrrrgh.
Or finding out a beta reader is right about some major story issues, and you might as well toss up the whole card deck in the air and start right over.
Or that your beta reader is right on some things but totally clueless on others, and you actually are looking at a totally different piece of advice than you thought you were working with. Like you completely did not realize one of your best friends was so appallingly blank and ignorant on a touchy issue you'd assumed they knew all about, and it changes your entire world view about them and about *how you pick out your friends.*
Or fighting with them-- people leave fandoms because it's so painful to keep dealing with that person they fought with, or keep fighting with.
That kind of cognitive dissonance is outright painful. For days, because your entire view of people and how they tick and what you do about them is in a heap like a Rube Goldbergian thing that the cat shoved off the table.
"Ooops, where you working on that?"
It takes awhile to sort out where your prior world view was inaccurate, and tinker with it. It doesn't even matter how trivial the actual trigger may be, the total flipflop is so massively distracting.
For me, that kind of stuff stays with me for *days.* It can have an arc to the stress, just like running a fever.
It takes some real managing to keep it from occupying my brain when I need it for other stuff. You know, like, driving cars, and work. Walking and chewing gum at the same time, stuff like that.
I don't know if you're asking for advice on managing the pain, but hey, every little management idea helps, I think.
I've been forced to handle it the same way I handle panic, or intrusive, repetitive thinking about some upcoming dreaded event: Set it firmly aside, tell myself I'm not working on that right now, and turn my attention on Something Else that needs doing. It helps if it's tangible, physical, real. Take out the trash, clean the catbox (phew! that'll get your attention), get the hose out and water the plants. Pet the darn cat who threw everything on the floor, perhaps. Now, if you've been doing that set-aside bit for 2-3 days, little bits have kind of settled out of the air, maybe you've talked to some folks about it, things are calmer, you can see the icky jagged bits where your map of the situation just cracked and broke right off.
Also, while I'm Not Thinking About That, maybe I've collected some random info on the problem, picked up a bunch of little notes, started a research file to start pushing out the debris and getting moving again. When I'm ready, I'll start piling that stuff together and see if any of it holds together.
And if it's still too much, I just go back to playing Cleopatra with the palm fronds for awhile longer, get that Rowing Down De Nile going again.