Has she had any testing done? Public schools will do it, or you could hire a neuropsych. It’s worth looking into. (There is no legit test for ADHD-just a questionnaire). Another idea, which was suggested to my daughter and helped another CC poster a few years back, is Irlen syndrome What is Irlen Syndrome? | Irlen Syndrome Foundation The remedy is colored glasses, which help make reading easier.
I would not mention any of this except for the way you mentioned she can get “completely overwhelmed.”
Another possible explanation for being overwhelmed is that she is doing too much too fast academically. Or the combination of training and ambitious academics is a strain.
I just wonder about a kid who is entering the second year of high school, feeling that she has to get started on AP physics and bio during the summer.