If you self-identify as autistic, people will doubt it because you don’t look autistic.
If you self-identify as autistic, and you write hundreds of posts explaining why, people will doubt it because you don’t have the medical establishment’s seal of approval.
If you get officially diagnosed, and it happens easily, people will doubt it because they’ll say the process wasn’t thorough enough to be accurate, and it’s becoming too easy for anyone to get a diagnosis.
If you get officially diagnosed, and it happens through blood, sweat, tears, and red tape – through an arduous and complex process involving your doctor, your family, and your school – and eventually getting legal access to accommodations makes you feel like an immigrant finally becoming a citizen – even then, people will still doubt it.
At some point, enough is enough. At some point, you have to abandon the urge to justify yourself further, because it’s a moving target.
If you find comfort and joy in the same things that give autistic people comfort and joy – including a sense of belonging with others like you – then no one can take that away.