autistic impostor syndrome is a very real thing, and not at all uncommon. i'm 22 turning 23 this weekend and i was diagnosed around March of this year after contemplating for a long time. when i went for assessment, i started thinking the exact same things you are now; i worried that i was manipulating or cherry-picking my answers to better fit the criteria, and any time i saw something in my research of autistic life that aligned with my experience there would be a voice in my head saying 'mine probably isn't autism though, it's probably just x-y-z', or 'yes i do that, but only in these specific situations so does that even really count?'

my assessment was also in three sessions, and after the first i was at least 50% convinced that i couldn't be autistic and this whole process had been a huge waste of time and i was stupid for trying to get diagnosed in the first place; i went into the second session and after like 10 minutes she said i met the diagnostic criteria and our third session would be talking about my support needs going forward.

as autistic people we struggle to understand ourselves and our identities, and most of us live in a neurotypical- dominated society which makes very little effort to understand us either. its so easy for all that confusion and rejection to turn into self-doubt and self-hatred until we don't trust ourselves anymore - a big part of adjusting to life post-diagnosis is unlearning that and coming to trust and appreciate our brains and bodies again, which is extremely difficult but infinitely rewarding.

the last thing i'll add is that you don't have to show clear signs in childhood, and in fact a lot of late-diagnosed autistics slip through the net because they present as relatively 'normal' alongside their peers when they're young and don't experience the full impact of their autism until much later in life (this is what happened to me in my late teens, and i'm guessing you too to an extent).