For me, the most effective way to deal with autistic imposter syndrome is to spend time in online autistic groups like this one, and to see for myself just how different people's experiences are.
And to do some work uncovering where the pain is coming from. Is it fear of appropriating an identity I might not have the right to? What does that mean, exactly? Who would I harm? Who would judge me?
Am I worried that I'm somehow "faking" being autistic because I'm too lazy or weak to face up to my flaws? Where is this judgemental voice coming from, in my life? Who taught me to be so cruel to myself?
And I find it helps to say "I'm autistic" as though I believe that I am. After a while, it starts feeling true as well as being true.