I think your mom is not communicating so well to motivate you towards better grades if she says you are a horrible student, which you are not from your grades, which are decent, but not particularly good. There is room for improvement, so if you would also like to improve your grades, you can try to work towards better grades (maybe with tutor), and don't resent your mom even though she is being insensitive.
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Drain and plane and grain and blain your brain, and then again,
Propane and butane out of the gas main, your blain shall sustain!