What to do when your parents never think you are good enough?

In absolutly everything I do, my parents, (my mom especially), tell me that I’m doing it wrong or not well enough. She is never pleased with what I do.

Example 1: If I’m getting a B in a class, (which is considered a little disapointing in my nerd of a family), my mom is disapointed in me. When I bump it up to a low A, she is still disapointed in me for having a LOW A.

Example 2: I’m sitting at the dinning room table with my family. My mom tells me to sit up strait. I groan. She yells at me for being disrespectful, so I say sorry. Then she tells me to cut my food into smaller pieces. I say I will. Then she yells at me for saying that sarcasticlly, even when I REALLY wasn’t trying to say it sarcasticlly. Then she comments on how I have a zit on my face and need to wash my face more. You get the idea.

Example 3: I help myself to seconds at dinner because I was just at volleyball and am really hungry. My mom yells at me and says if I keep eating so much I’ll get fat. The next day I’m not hungry at all at dinner, so I don’t finish what is on my plate. My mom yells at me for being ungratefull and not eating what my father has cooked for me. Then, an hour later, she tells me I could lose a few pounds.

How can I explain to my hostile mother how difficult she can be and how crappy she makes me feel about myself?