Sonny40,
Please do try not to worry (pointless saying that, I know, but still)
I too am a mother to an ASD child. Dd1 is nearly 6, and severely autistic - probably exactly the level you are worrying about. She has severe delays, and attends a special needs school, with a full time 1 to 1 assistant. I worry everyday about her future, and how the world will turn out for her when she grows up and we are not here anymore to look put for her.
But I have those very same fears for dd2 as well. I do not know that she is going to be perfectly happy, or that everything will run smoothly for her in whatever life she turns out to have. Dd2 is not disabled in any.way, but my fears remain nonetheless. Of course I hope she will be happy in every way, bit I don't know that this will be the case. Parenting is full of uncertainty, whether your child is disabled or not.
When you have a child, you do assume that all will go well. That life will pan out nicely, and that everything will be fine for your child. It is not wrong to assume this, nor wrong to worry a out how it might go wrong. As you say, it is all part of a fear of the unknown.
But, like gigantaur, I promise you that come what may, you will love your child. I too was completely unmaternal before i had my dds - I'd barely even touched a baby before that point! But they arrive, tiny little scraps that they are, and suddenly the whole world shifts a little, and the new focus is this tiny snuffling little thing...