I definitely agree with others that it's about forcing yourself to fit it and using a lot of energy to do so. It's generally a conscious behaviour that isn't maintained in your safe spaces (like at home).
It's effectively acting, you learn how you should behave even if it makes no sense and so act accordingly. "Masking" your true self.
I am undergoing assessment for ADHD and ASD and score highly in all assessments for both. I have sensory, routine and social issues from the expected ASD.
But you wouldn't know it without me telling you. Particularly the social side.
But that's because I've watched hundreds of hours of documentaries on brain development, child development and social development. I've read booked specifically in the art of understanding body language, facial expressions and how to influence people. I've also completed several courses that are around soft skills and emotional intelligence. It has always been the one thing that gets brought up repeatedly through my life so I actively sought to change it.
I still don't get why I have to smile, make eye contact, small talk, take an interest or anything else I'm expected to do. Especially at work... We're here to work, can we not just get on with it rather than doing this silly dance?
But I can copy it all very well now. It just requires a lot of concious thought all the time. And leads me to situations where I'll come home and just sit in the dark in silence once the kids are in bed and wait until it's time to sleep cause I just can't face anything else.
If I had a choice I'd never leave the house or socialise with pretty much anyone except the odd digital friend and DH. But I accept that you can't survive this world alone so you have to put the effort in and accept the sensory onslaught of going out and the mental exhaustion that goes with it and the socialising.
Also the sensory and routine issues. I have ANC ear buds that I use to block out sound. Perfumes I used to block out external smells. I will actively lower my eyes if I'm in a busy environment that's causing me to struggle to think. I actively avoid going somewhere new, and if I do go anywhere new I will rehearse the journey several times on Google maps and probably run it a few more times in my head as well as trying to predict any potential disruptions that could occur. You've also not seen me crying just because the towels aren't folded right 😬. Or when I'm rendered speechless and unable to think because the takeaway order wasn't in the packaging I expected so it was "wrong" and I couldn't eat it. None of that is stuff I'd say I "mask", but it is hidden. Cause unless I told you then you wouldn't know.
In reality people just know I can be a bit flakey and I'll normally say no if they invite me places. And that the explanations I give are pretty weak. It leads people to assume I don't value them or care. I do. But not in the way they expect. If they needed me to go and help them at crazy o'clock in an emergency I wouldn't hesitate, but ask me to go out to the pub for a meal and I'll probably ignore that text for as long as I probably can 😅😂.