I hate my grades why can't i understand lessons quicky?

It hurts so much, I just can't believe it it doesn't matter what i do i always mess up! I hate myself so much, i can't believe it a whole semester of work and it's gonna go down the drain because of a failure i made! I'm still a freshman, i really want good grades because colleges are expensive and i want a scholarship. I know grades don't define me but it hurts so much! All the effort i made! I admit i'm not that smart of a kid i'm an average kid, i'm still learning a lot. I want good grades so why does life hate me so much??? I have an A- in biology but i really love the class, but i just recieved that the presentation I did got a C. I know i did shitty but i didn't think my presentation was that bad. I feel so overwhelmed because i know when my teacher inputs that into my grades i'm gonna drop a lot. I don't know what's wrong with me. Maybe because it's almost summer break, i want summer but i also don't want to slack off but lately i'm messing shit up and i hate it. I just want good grades, this hurt so much. I know it's just grades but i just want to make my parents proud they sacrificed EVERYTHING for me to get into a good school i don't want to dissapoint them. I already did i want to make it up to them i just feel so overwhemled right now and i feel so stupid and angry at myself why can't i be smart???