Does your mother know about your eating disorder? That is, does she admit she knows?

What she's doing is very damaging, as you know, and the best I can recommend is to find a good therapist who knows enough about eating disorders that s/he will recommend family sessions. Your mother knows, on some level, that there's a problem. Why she chooses to make the problem worse, I can't say.

And you do know that you need to be eating more, right? You should never get below 1200 calories a day. (Yeah, I know -- I'm anorexic, too. I rationalize a lot of my crap by saying I'm middle aged, I don't have anything to protect, to look forward to, etc.) You've got too much to look forward to. I wish so much I could go back and do it over. Life doesn't give do-overs. Get yourself help now. It really will be rworth it for you.