One thing that has helped with my daughter is to talk to her when she is not in the middle of schoolwork and is calm about how being wrong and then figuring out how to fix your answer is the way your brain grows. Then when she is melting down about not being able to do something, I can tell her that the discomfort she is feeling right now is the feeling of her brain stretching and growing. It (sometimes) calms her down and gets her back into working on the problem. It probably also helps that she heard Richard Rusczyk speak last year about how if you know all the answers, you're in the wrong classroom. It was more convincing coming from him than from mom.
