From my own experience, and what I've seen online, there is a permission issue post diagnosis.
You've been masking for years without knowing it, exhausted and irritable on a seemingly irrational basis, and if something else happened in your life to kick off worsening symptoms (age, sudden life changes), it hits you all at once. The diagnosis comes and goes, the imposter symptoms come go and come right back, double imposter syndrome.
This is where the permission comes into play, giving yourself permission to stop and not perform at the highest levels, not to be a perfectionist, to deliver tasks and results at the same rate as the baseline of humanity. The permission to realise what's going on and the hidden autistic costs inherent in every day things and events which neurotypicals just blithely breeze through.
Give yourself guilt-free permission to leave social gatherings a little early - let people know in advance, less guilt. Note to yourself that before appointments or meetings there is a period of autistic-preparation, you're not just waiting, you're spinning up 17 parallel universes of all possibilities, going around in a loop checking your facts / work / preparation. Invariably for me, the appointment is nothing like any of the 17 parallel universe outcomes and smacks me blue into next week - acknowledge this will happen and plan some quiet recovery time, say no to going from one thing to the next, and limit the number of things you do in a day.
There's said to be an autistic calculation going on all the time, it's not the same as the neurotypical calculation - allistics / neurotypicals work in time, do they have enough time to get stuff done? Autistic people work in energy first, and time second - that 40%+ extra processing you're putting in without planning recovery, that's borrowing energy from tomorrow, and that leads to burnout.
There have been situations where I've had to push through and borrow from tomorrow, it happens, life happens, but it's a debt which I'm building. Before the diagnosis I was just doing this, and being exhausted. It's one of the things I got gaslighted about a lot, I'd complain that I need to stop now or I've gone beyond by limits, and got smacked in the face with 'everyone does this', and feel unable to push back, and ended up in persistent chronic burnout. Permission to admit that I can't do that, and being firm with others is starting (eight months post diagnosis) to kick in.
The phrases 'everyone does this', 'everyone goes through this', 'everyone has to muck in' are lethally toxic sentences to an autistic person trying to push back against neurotypical demands. The unintentional gaslighting really makes me angry. Giving myself permission to call it out has been a game changer. I know that you may be trapped by circumstance, so be gentle with the neurotypicals, they really don't know, and it's incorrect to think they should without being informed.