I can't advise you as a parent, Hurlshot, and even if I could I wouldn't regard myself as superior.
However, as a manager I'd advise you to look at building up the subordinate 'building blocks' of the functioning behaviour. Make her stronger, give her more motivation to complete, positively enable prior through a prep (but don't prep by doing the actual performance, do skills affirmation). I'd also consider whether it's her thing to be performing at all. It's not in everyone's character to want to go out and get public applause or derision, and sometimes it's simply because a person knows they have all the interest, care and support they need already.
EDIT: also, as an amateur actor and singer I'd particularly single out the pre-prep. Do stretches or vocal exercises beforehand to prove you've got the chops to do the full show. BUt don't do the full show, because then you'll get complacent.