How do I deal with my mom about this?

So I’m having stomach surgery and my surgeon said it is best if I lose weight so he can make my pouch easily. And so I’ve been trying my best to lose weight. I’ve lost 30 pounds and now that my surgery is very close ( less than 2 weeks ) she is going ham on me. I only eat once a day. And I go to the park everyday. She gets mad because I eat something “ unhealthy “ and blames me for eating something so small. And gets mad at me for eating after I come home from the park. Like i know I need to lose weight but I’m trying. My sister always tried to defend me and tells her something but she always tell her not to put her spoon in things she doesn’t care about. It’s just really stressing me out how she has complete power of what I eat. How do I deal with her?