I feel it is really important for you to talk to a counselor at your school about this, and get an outside opinion. You need to separate your thinking from your mother's thinking, which is authoritarian, cruel and unfair towards you. You and she are not the same people, and you do not have to be exactly as she says. A parent should be authoritative and make sure that kids do their best in school, not go nuts when they get a 3.8 and not a 4.0. Your mom or dad might feel like you need to be successful maybe because they truly care for your success and future...I'm just making educated guesses here. But it's not OK to put that amount of negatively stressful pressure on you.

It's amazing that you brought yourself up to having mostly A's from having C grades. This is a huge accomplishment and this alone shows how much more seriously you take school - and your parents should be happy and proud of your for it. Unfortunately, not all parents are authoritative, understanding and rewarding as they should be. Is this your fault? Not at all. I feel like, even though it'll be challenging, you need to distance yourself from your parents' expectations and be very assertive about your own accomplishments and how successful you are. I think a school counselor could even bring them in and discuss this with them, with or without you there, if you're comfortable with it.

Good luck.