Why do I study so so much and still fail?

I'm in grade 12 and currently in my second term(of 4 terms) and well last term I studied so hard and the results reflected those of someone who did not even bother studying. I would spend 2-4 hours (Monday - Thursday) mastering what we did that day and on weekends I would study a total of 24 hours on 2 subjects per weekend. I would of course study until I understood but that is just an approximate estimation of the time spent.

Honestly I just feel so defeated. It feels as though there is nothing I'll ever amount to...and the pressure from my family just makes it all worse, with my cousins getting As and Bs in their 12th year last year, one graduated from university and the rest studying Medicine and such...I mean I would like to study Psychology next year but it doesn't even look promising at this point. My parents are utterly disappointed at my results and I can't blame them. I just need to understand why and how come it's just not working out for me...suggestions will be greatly appreciated.