{{{{{Pat}}}}}
That has got to be so difficult for you, not to mention hurtful, especially how you helped your daughter get through he addiction.
You said she reads a lot and I got the impression is knowledgeable on Bipolar illness, so that doesn't seem to play into it.
I would agree a big factor is fear. Fear of losing you; fear that you wouldn't be able to help her if she needed you, who knows, all kinds of reasons.
Sending warm thoughts to you....