I think my daughter is stressing too much

Oh dear - this is becoming all too common.  I suggest you have some conversations with your girl to figure out where she is getting the "information" that she builds anxiety around.  I see teens  who have heard teachers, parents, coaches, etc push the "highly selective colleges" idea on kids way too much.  Teens in particular are prone to black and white thinking, leading them to see one poor test performance, one off day, one not-great assignment as the make-or-break task on the path to successful adulthood.  If that's the case with your daughter, she needs you to help modulate both the incoming information (there are nearly 5000 colleges in the US - she will get into one of them!) as well as her reaction to that information.

As well, you and any other adults that are involved in parenting her need to discern fact from fiction.  The recent scandal around celebrities "buying" their kids into college is an indicator of how far off track we, as a culture, have gotten in terms of education.  A college education is certainly important, but we have allowed the marketing push to create so much stress and upset for our kids (and ourselves as parents) that they are actually less prepared to leave home and really engage in higher education than ever.

You may want to include a therapist in these conversations if you get stuck, but start with a parent-child conversation.  Help her figure out what is going on - if it is the "I have to get it right or I will have no future", calmly walk her through all the evidence that says that's just not true.  As well, let her know that being anxious now is NOT preparing her for a bright adulthood - it's preparing her for an anxious future.  She needs help figuring out how to balance her effort to attend to real and reasonable expectations, not over-inflated fear-based actions.