My girlfriend criticizes me a lot, she won't stop and I don't know what to do?

My girlfriend criticizes me a lot because I study Engineering and Mathematics. She starts complaining that I am too much of a "nerd" and that I am working on nerdy things all of the time -well, studying those things is actually very time consuming, so there is no choice.- and that she does not like a "nerd"

She also complains about me being overweight all of the time, she pretends to give me exercise advice (which she really could not do, and actually is pretty ridiculous advice. I've taken enough classes about it and read enough to know.) And I am not really overweight, just a bit, and I do a lot of weigh lifting, so I am actually very muscular. Just need to loose some 15 pounds of fat and will be fine.

She keeps complaining about so many things. She even complained that I am too white for her, that she wants someone darker, etc.

She always finds things to complain about me, I don't know what to do about it. Its driving me crazy. It seems there is no way of pleasing this woman, what should I do?
My girlfriend criticizes me a lot, she won't stop and I don't know what to do?
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