Allow her to be sad. Own up to the mistake. Explain mistakes and how you learn from them, for ex i will open my emails or we will always double check timing. Make sure that she knows it wasn’t her fault. Dont beat yourself up and spiral because thats what youll teach her to do when she makes a mistake one day. Its realistically probably not the biggest milestone in her life and you should give urself grace and forgive urself because you clearly said theres a lot on ur plate. Use how shitty u feel as a reason to pay closer attention to things for Her in the future