I haven't experienced this particular problem before but I can offer my thoughts.

If the current work situation is becoming untenable then I would perhaps start looking for a new job, however do not walk away until you have secured employment somewhere else. I'm getting the impression you have tried going down all the available avenues to rectify the problem without having much luck. I honestly believe the worst thing you can possibly do is let the cow and her cronies know that they are getting to you. If they become aware that they are getting to you they will thrive on it. There is an old english saying that I think pretty much says what I want to say "Don't let the bastards grind you down". I know It sounds very easy, but I think this is what you have to do until you either change workplace or she disappears. In the meantime just try to do your job the best you can, they cannot fire you without a good reason so don't give them one.

Do not let these people make you walk away from your job, because it will cause more stress about your financial situation and they will just line up another victim behind you.

May I ask how you usually manage stress and anxiety? do you meditate, take medication or simply just try to tolerate it? as to the stomach issue if you are experiencing IBS symptoms a doctor may be able to prescribe you an IBS medication such as mebeverine.