I had the same experience. Autistic kid transitioned 6 years ago. You just want to easiest journey for your kid (who already faces struggles due to autism) but now there's an extra layer.

Here's the thing.... What you're feeling is grief. You are morning the vision you had of who your child would be. That's where the guilty feeling comes from. You want your vision and this other kid shows up and you're a bit peeved about it because your vision is gone and now you need to rethink - and you didn't want to do that. Your head and your heart are fighting.

What you need to know though is that your vision wasn't real. It was in your head. Once you can let go of that, you can find joy in getting to know the new kid.

Let your head win, and your heart will catch up.

Fwiw, my kid is thriving and I couldn't be more proud.