Yes, it’s the perfectionism. It can be paralysing.
I think you need to dial the pressure and expectation RIGHT back.
I actually think I might withdraw her from sitting the exams altogether, depending on you think ‘failing’ would do to her?
Failing could, for example, on the positive side:
-Relieve the pressure, help her to realise the ‘worst case scenario’ actually isn’t so bad. Once she has failed everything and found that life goes on and everyone still loves her, she may feel liberated enough to relax and do re-sits.
-It might also galvanise her into action, wanting to prove to the world that those results aren't the best she has to offer.
OR, less positively, it might:
-Diminish her view of herself, confirm her worst fears that she is a failure, that life is too hard, and cause her to retreat into herself.
I think it hinges on what her self esteem and mental health is like. If it’s low, I’d be tempted to excuse her from exams, let her take a year out doing something productive that she’s good at (outdoor education stuff like outward bound is amazing for building confidence and getting out of her comfort zone, if she’ll do it). And look into her doing GCSEs at college the year after, when she’s feeling more positive and the pressure from school is no longer present.