Autistic imposter syndrome
As autism's pretty rare, many autistic people — especially those who can't afford to get an official diagnosis — doubt themselves, even when they intellectually know that it's the best fit to explain their whole life.
I think bearing in mind Occam's razor helps. You have to make a shift from "What are the chances I might be autistic? It's so rare!" to "This is the list of traits that I have. What are the chances they all have independent, unrelated explanations, compared to the one simple overarching explanation of me being autistic? That would be improbable!"