How to Handle Anxiety Over Cleaning Demands from Fiancé's Mom?

You guys are probably gonna think I'm like ungrateful, but, I get so much anxiety when my fiance's mom tells me to clean. If it was just my mess it would be fine, but she also yells at me for everyone's stuff. I get yelled at for things I didn't even know about. And yelled at for not knowing it. Most people have to clean, and I get that, but I have PTSD regarding cleaning (long story, but to sum it up, as a child, when I wasn't in school I was cleaning. That is not an exaggeration.) Sometimes I just roll with the punches like anyone else (though I feel nauseated), but sometimes I have meltdowns, attempt to leave (again, you guys are gonna think I'm crazy) and it's this big thing, and I feel like crap after for exaggerating. IDK what to do about it. I can't tell her cuz she probably wouldn't believe me, she says it's not her problem when her son says something about it. I don't want to leave, but I'm at a loss on what to do. (Besides therapy, cuz I'm not great at keeping appointments) what do you guys think I should do?