My education is killing me... and my mom is making it worse, what should I do?

I used to get good grades in school, usually A's and sometimes B's, but now they have gotten much worse. They are mostly B's now. I still try hard and study everyday, but I can never concentrate and nothing sinks in. To make things worse, my mom is ALWAYS pestering me about it. Every single day I come home, she is always whining or fussing about it. My grades are bad enough as it is, and now I have to deal with so much unnecessary stress and now I'm getting constant headaches. I can't even be free in my house anymore because of how much my mom is pestering at me, and most of the time I just lock myself in my room... until she comes banging on the door and giving me a headache again. But what's really annoying is how whenever I say that I'm getting annoyed, she says all sorts of garbage about how I will never succeed in life and how I am a complete failure. Please give me some advice, and thank you in advance.