Imagine you speak French as your first language.
In your case, this is strange because nobody you knew growing up spoke French. You are surrounded by people who speak English. Your family speaks English and only English. You’ve never been exposed to any language other than English.
Certainly not French.
So you don’t know how you speak French. You just knew that, as a child, you didn’t know how to speak English. Everyone around you just nattered on and on and you didn’t know what they were saying.
You did learn to speak English, eventually. It’s not your first language, it doesn’t feel very natural to you, and it’s still fairly difficult. But you get by.
And since nobody around you speaks French, and you don’t have any materials in French, and you don’t even know that what you’re speaking is French, you don’t know how to even look for resources. You just suffer along in this strange language in this strange world that everyone else calls home.
The longer you go without finding other people who speak French, the more convinced you become that you’re utterly alone in the world. You get used to thinking that your language is utterly unique, and you get to the point that if you ever did meet a French speaker, you’d think they were either faking it or mocking you. Maybe both.
This is what it’s been like for me as a late-realized Autis.