There is one thing you should try to do before going to your boss—come up with a solution to rectify your mistake (along with a few alternatives for her to consider). If you can't come up with something quickly, go to your boss anyway, but reassure him that you are working on some solutions. Once you know what needs to happen, present your plan clearly. Tell your boss how long it will take to implement it and if there are any costs involved. Be ready to also present your alternative solutions just in case she shoots down the first one. Problem solving is an important soft skill.