Wow your situation sounds a lot worse than mine, just because my life isn't under threat. but I can relate somewhat. I'm thinking of applying for a teaching job here in my hometown, but I'm hesitating because some of the classes they want taught are ones I didn't do so well in myself. My advice would be to be honest about your mistake next class period. I have had many teachers make a mistake and come back a day or two later and correct themselves. If the material you are teaching is complicated then you're bound to mispeak every once in awhile that's what makes you human.