Why have I become so lazy?

I know it seems like I just don't care but I do. Lately I have become really lazy and my grades have been taking a really hard hit. When my grades are bad I feel like I suck and I'm worthless and all I want to do is just go to sleep and forget about everything because only when I'm asleep I forget of how much of a failure am I. I look around at my classmates and they all seem to be doing fine and understand what's going on, then I look at myself and feel like i'm an idiot. I've always been the smart straight A's student but now I've been really falling behind for some reason. I am always really stressed since I have to excel at every single class at school if I want to get all A's and get into Med School and lately I've been thinking i'm too stupid to actually succeed, the MCAT just seems far out of my reach and yet I need to learn all of it and do great in it if I ever want to feel better about myself. Sometimes I want to change this and just study all day but I can't. Even if I slept all day and night I just don't feel like doing anything at all and feel like if I was very tired. It's not that I want to do something else like watch TV or play videogames, I don't even feel like doing any of those things that used to be fun at all anymore. I just don't feel like doing anything at all, boring or fun makes no difference. I don't feel motivated and I feel like i'm worthless and too stupid to achieve what I want yet when I try to change it I only end up feeling worse about myself because I tried and failed which makes things worse. I don't know what's wrong with me. I really need help to know what can I do. Please help me with this.