For me (ADHD), apart from the obvious things - sensory issues, etc, which as you can say you can picture relatively easily I think - it's a sense of wrongness a lot of the time.
I very often feel uncomfortable and not right and as though I have to try very, very hard to be 'normal'. Even alone, when I've no need to mask my behaviour, I cannot watch TV without shaking my legs or being distracted or missing half of what's being said because a tiny noise in the next room has hijacked my entire brain power and I am acutely aware that I can't just sit down and watch TV like a non-ND person.
I can't socialise like a neurotypical person because it gets to a point where it feels physically painful, like having raw skin and nerves and everything is rubbing on them.
I can't keep my home tidy, keep things organised, remember anything. It sounds minor, but when you are overwhelmed constantly and you collect parking tickets and late payment fees like Pokemon cards it compounds the guilt of not being like a normal person, and not being able to explain that "just put an alarm on your phone" won't work because I will immediately cancel the alarm because the noise is stressful and then just as immediately forget.
I hate that I can't "just do" things. Executive dysfunction has been the most difficult and defining element of my life and being diagnosed in my twenties set of a slow avalanche of "oh that's why I couldn't do X Y Z" that even now, just hurts to think about. It's why I couldn't "just start" my dissertation (getting me a terrible mark for my rushed-through, 1 week to go, adrenaline driven effort), it's why my front garden is a mess a year after saying I needed to repaint the bench and tidy up, because I can't "just do it". I can be sitting on the sofa wanting to go for a shower before bed and I just... can't. I'm not scared or physically unable or anxious, I just can't get the linking bit between thinking about it and actually doing it. And that is something I struggle with every day - work that needs doing to a timescale, not when I have the random spurts of 'able to go'.
It's a very shame-inducing feeling, and something I have to work hard not to internalise as much as I have.