It's Okay to "mess up"!
Everyone messes up sometimes. Often multiples are put in the positions of having to be paragons of stability, functioning, and judgement. They are expected to be so 24-7, else they be branded non-functional multiples. This is an unfair, and arguably impossible standard. Nobody is always stable, or together. Nobody always makes the right choices in life. Nobody.
Everyone makes mistakes. It's all in how the person handles it when they have.
These are the things that give us "character".
Working through the mistake
Well, for starters, try not to be too hard on yourself about it. This isn't always easy however. Berating yourself won't help you. However societal programming runs deep, so you may find yourself having trouble not doing so. If that happens, remind yourself that although you made a mistake, you are working on the situation, and work on it. Focus on working on the mistake, and it's repercussions, not on the fact that you made the mistake, nor on the idea of fixing the situation -- that's another trap people fall into.
Depending on the circumstance, you may need some time taking a break from the problem. Some situations may not allow for that, or in large enough quantities. In those cases, when you can, try to take a moment to relax, even if only for a couple of seconds. Take a deep breath, count to 10, think of your favorite color, whathaveyou.
--Arashi (w/ assist)