PTSD and difficult boss?

Please, if anyone has any experience with this give me some advice. I have been diagnosed with severe PTSD, neurocardiogenic syncope(stress makes my bp drop) and low blood sugar. I work hard and can get through the day fine by coping mechanisms. However... My boss has a huge issue with me taking a short break( even 5 min!). I get there at 8 and work til lunch at 1. I'm pretty sure it's the law to allow a break. Without food my bp and bs both drop so I have to sneak to the bathroom to eat. Kind of gross, I know, but no other option. But also my boss is a control freak who loves to undermine you and criticize. You never know if the rules or instructions she tells you today apply tomorrow. She has thrown me into panic attacks more than once with her "lectures" that never make sense. Like one of my new assignments( I work at a medical center) is to always smile. She said she is aware I smile when I seat patients and while I'm in the ops but that if a patient happens to look out the door they need to see a "happy","carefree" employee that thinks their job is wonderful. If she catches me not smiling while working like an insane weirdo I am in trouble. Sounds small, I know, but it is something like this EVERYDAY. I try to just agree and do what she wants so peace remains but I am about to reach the breaking point. Ive tried discussing this things with her but wind up locked in her office for thirty mins to hear why I'm in the wrong... And yes she knows about all my health issues. What to do?? I enjoy(or can deal with) my job in every aspect other than her.