I do have some sense that this is about existential angst/depression and some sense of isolation due to being PG, plus that general disconnectedness that many adolescents feel (and which seems to be something of a developmental thing).
In Dabrowskian terms, I'd say this is a disintegration in progress.
Not sure what to do about it, however, or how to offer her any guidance. Not even
sure that a mental health professional would have very good guidance if I'm right about causation.
The Zen suggestion is a good one. I think that I know precisely what that means, actually...

But I'm Taoist in philosophy and have been for decades, so Zen often makes a great deal of sense to me.
I think that guided meditation, Zen (or related) quotes/philosophy, and art-therapy (again, the DIY variety where DD retains autonomy of the process) might be a good way to approach things with her.