I don't have kids but part of my job is working with teenagers so I do have a decent idea of what it's like.

Some of the self-talk can be normal. A lot of teens are looking at their lives and the 'fantasies' of who and what they want to be. This can often involve self-talk and could possibly be the cause. Most grow out of it by 15-16 years.

If it is ongoing though or those daydreams etc. are causing ongoing issues with her school and routines, which it seems they might be, I would keep an eye on it. Maybe also mention it to the doctor just so they are aware.

The hard thing with teens is that it is almost impossible to force them to open up about their thoughts and feelings. She will need to get there in her own time. The best thing you can do is just continue to be supportive like you are and let her know that you are always there for her if she ever wants to talk and that you will not judge her for what she is feeling or thinking.