Hi Candy,
I'm not trying to minimize your situation, but one mistake doesn't mean you're a big loser. People get the wrong location, time, sometimes even day wrong all the time. I think the difference is that people with mental health concerns tend to be their own worst enemy. It was a mistake, it's not as if you weren't there sincerely to get the job done.
Is there anything you can do today to rectify the situation? If yes, then do it, get it over with, otherwise you'll ruminate. If there isn't anything you can do until tomorrow, I would try to put it on the back-burner, since torturing yourself isn't going to serve any purpose other than to make you feel worse.
I can feel your frustration just by reading your post (the head bangers gave it away, go figure.) I hope you can be easier on yourself. It was a misunderstanding.
Please take care!