It sounds so frustrating. She seems to know herself very well. I had not thought very much about my own experiences with this; maybe I assumed it was normal. That she's able to recognize it as significant makes me think she's pretty perceptive.

Does speaking exhaust her? I am wondering if perhaps she could sometimes use a back-up method of communication like sign language or picture cards, something that is a little more visual so that she doesn't have to constantly push herself to use words. It's a little like how you might, if you had problems walking, occasionally use a wheelchair or a scooter so that you could save your energy and get places faster--not because you can't walk, but because it's so exhausting.

I have had some similar issues with having to watch my own mind change. I was, from an external perspective, "more autistic" when I was younger. As I gained more ability to socialize and use abstract language, I lost some of the ways of thinking I had back then. My sensory style has changed, gradually. I used to notice every detail in the world, remember everything. Now, sometimes days and weeks pass in a daze. It's like I'm half-asleep. All those little details that used to flood in... now, some of them don't even enter into awareness. And yeah, I mourn that. I can't help it. It's like I've lost part of myself. It feels like time is passing faster and faster, and life is less and less vivid, even as I'm becoming more independent.

I don't know how you can help her, really I don't. But she's not alone. Parts of being autistic are beautiful, thrilling, enchanting, and you don't want to lose them, because they are part of you. And when your mind changes anyway, because you're growing up, you mourn the loss, just like you might mourn the loss of vision or hearing if you had lost those. At least, as she deals with this, she'll have a person who cares about her and wants her to be happy, who understands that this is a real loss and not just some "behavior problem" that has to be squashed. It's good to have family on your side.