I understand totally. That's the problem with 'highly functioning' autism. It's perceived that we function really well, when in fact most of us struggle with the most basic aspects of existence that most people do without thinking. It takes a lot of mental effort and life can very quickly overwhelm us - which is why most of us have a string of mental health disorders and/or chronic illnesses.
I realised recently that I
was happy until the age of 5. I was in my own little world - nobody bothered me - and aside having to go to the sensory nightmare that was the hairdressers with my mother or sit on crowded buses which made me feel sick and relatives coming to the house - I was
happy. Then I started school and it went downhill instantly. I was in the wrong place with people who didn't understand me, nor I them. But it was the 70s, and there was no autistic spectrum - me, and children like me, were punished or ignored.
It is what it is, James. You cannot change how your brain is wired. My son is autistic too, and I am encouraging him to be authentically 'him' - not a second rate NT version so other people feel less uncomfortable around him. He's starting high school now. He just learnt to tie his shoelaces. It's been an awesome week. He teaches me a lot too, and he is the happiest child that I know.
I go through dark phases, but it's predominantly anxiety - always has been - but I have been clinically depressed and I'd take feeling 'plugged into the mains' any day over the feeling of 'nothingness'. At least with anxiety, I feel
alive. But it is exhausting, and my body agrees - which is why I have the biological age of 209.
Have you looked into dissociation and depersonalisation?
Things are less scary once we understand them and the one thing that most autistics do well is to research!