i just can't anymore

i'm literally breaking down over school work right now. i've always been the smart kid and my future hinges on the subjects i'm not the best at. i'm so scared of letting everyone down and disappointing those who have high expectations from me. i feel like i can't talk to anyone about this. the only time i matter is whenever i do well academically. my brains are literally all i have going for me and my mind is crumbling. if im not smart, im worthless.