Help!! Any suggestions??

I have this huge problem that is stressing me out to the point of constantly having headaches, and stomach aches... The "acting" manager where I work for some reason has it in for me. My shift is the one where all the cleaning and stocking work is done. In fact, there are people during the day who she requires to do  nothing more than running the register (and who, I might add, receive the same pay as I do) My shift is 8 hours long, but she requires all my work to be done in 4. Taking apart all the machines and cleaning them, sweeping the lot, cleaning all outside displays, even cleaning the payphones every night...doing all the inside cleaning, and much more... Plus running the register.... In 4 hours. If you just saw all the work I am required to do, you would understand. Then, when I have done it all to the best of my ability, and worn myself out, she comes in a starts complaining that I haven't done it!!! There are a couple of people I work with that can't understand why she has it out for me. I can't understand it either. Nothing I do seems to appease her. She took the weekend off, and another woman from  a different store came to do the books and stuff for her... She did the "managers store inspection" in the mornings, and told me what a good job I had done... And I did nothing different from the nights when the "acting" manager comes in... I don't get it!! I don't know how to deal with this, there is no reasoning with her. I am a single mom, and this is one of the best paying jobs in this tiny Hick County town, so I can't exactly quit... Plus the benefits are great... I'm just getting so stressed out that it is affecting me at home too, not just when I am working... I have had someone suggest wearing an amulet to work to ward off personal attacks.... Someone else suggested prayer.... I just don't know how to handle this... I can't go over her head, because the "big wigs" almost always side with the manager. The lady that came this weekend to replace her told me that the company is not happy with her performance... Maybe the best I can hope for is that they will move her into a different store, so she will leave me alone... I am working my a** off all night long, and all I get for it is raked over the coals... There is nothing I can do to work harder... I have even been taking caffeine pills to make me go faster... And she still isn't happy....