Have you considered taking an anti-depressant? I resisted taking them for a long time, because I didn't want to get hooked on anything (and there were many other reasons). Finally, I realized that this is more than I can handle right now. I have a two-year-old son who had a very grumpy mother, and I didn't feel like it was fair to make him to suffer along with me. In your case, maybe you need something to get you through the rest of the school year. It could just be short-term thing. I completely understand your concern about student teaching, because I did that my last semester in college. Its stressful, and its a lot of work.
If you do, in fact, have Irritable Bowel Syndrome, an anti-depressant/anti-anxiety pill could help. My doc told me that only 5% of seratonin is in your head, the other 95% is in your gut and is a big trigger for IBS. I just recently started taking Zoloft to see if it would help. My symptoms haven't shown much improvement, but I am definitely less stressed out lately.
Whatever you decide, I hope you can regain your positive outlook. As a glass-is-half-full kind girl, I know how shocking it can be to suddenly feel hopeless all the time. Good luck with your student teaching. I hope you can enjoy it as much as I did!