Why can I write backwards/mirror write naturally? I am left-handed and autistic.

I don’t know, but interesting you ask that. My eldest son (ASD 1 / high IQ / right handed, but ambidextrous) had that ability as well. He taught himself, but it’s didn’t take much teaching / practice. By 6 - 8 he was pretty fluent and quite legible, though his normal penmanship was pretty poor. I had actually forgotten about that. I don’t think it ever occurred to me to mention to any one. It just struck me more as a craft or hobby as that’s how he used his skill.