why is my mother so nasty to me, answers greatly appreciated?

we either get on really well other times we hate one another. she's lik jekyll and hyde. she wants everything to be about her and accuses me of wanting everything to be about me. she makes me feel guilty that i have depression and ocd and she isnt very understanding. she just yells at me saying she doesnt understand how i have depression when im 21 with a bright future! she tells me to snap out of it, she acts like its so easy. also she makes me feel really insecure about my weight. i binge eat from time to time because of stress though im trying not to. she's like look at you you've scoffed everything. im a UK size 12, sometimes a 14 and 5"7. I'm trying to get down to an 8-10. I was making myself dinner because i eat and she was like why are you having potatoes, they're fattening (i put a small amount of my plate) and she was like why are you eating, you had lunch? im like this is dinner! im trying to watch what i eat and today ive been healthy and danced in my room for exercise. the thing is she's overweight, a size 20 and she thinks she has the right to give me diet advise. i've lost weight before through exercise and healthy eating, not by cutting meals. i dont cut meals, i eat 1500-1700 calories plus i exercise 3-4 times a week! why is my mother so nasty to me? she always puts me down and is really patronising. it feels like i can never do anything right.