I can't pass calculus?

I don't know what else to do. I am pre-med student and every class I had ever taken, no matter how difficult I had been able to study and improve. I am close to graduating and my GPA is going to bomb if I get a C or fail my calculus class. I am usually an A or B student but in calculus I have failed all of my tests with a 50 or below. I just can't understand it, I try and try again and nothing. I study more than 12 hours a day during the weekends since my tests are on mondays and that doesn't even work. I practice, I read but when I get to the test I just can't take those methods directly out of my mind or know when to apply them, sure it's easy if I have my book with me or just did several problems a minute ago, but in the test not a chance. I am completely lost. I don't know what to do, no matter how much I study, how much I read or how much I practice I cannot understand a thing about calculus I don't know what to do. I feel like a failure, can someone think of something that I can do? I am desperate