What am I doing?

So again I am asking myself what am I doing? More like what the f*&% am I doing? I started this school year being very motivated and excited about the classes I was taking. And little by little it has all fallen apart. And it's only been a little over a month! I have two full months left.
I didn't do very well on my physics test. It made me really mad. I know I didn't study as well as I needed to for it but the test itself was actually very simple. I don't like the way the points were assigned. There were 20 questions, each worth 5 points. Doesn't sound too bad right? Except that half the problems were true/false. So one true/false question was worth the same amount of points as a problem you had to work out. I just don't like that. If you miss too many t/f questions you're already screwed and there's still half the test left.
I just feel like. . . I can never do anything good enough. There is always something keeping me back from doing as well as I know I can. But it's like I'm just not willing to do it. But I want to. I want to be able to study and do really well in my classes. I want this education I'm getting to actually allow me to do something good with my life. This weekend when I volunteered at the hospital it made me say "yes, I want to do this." But so much is dependent on my grades that I don't think I'll be able to.
Why can't I focus? Why can't I manage to do the things I know I need to so I can be successful? Is this God trying to tell me I shouldn't be going into the medical field? Or is it just the same things that have plagued me since I was young that are going to get in my way no matter what I try to do? Are things from my childhood (always being "smarter" than the school required me to be, so I didn't have to try hard, never establishing good study habits, etc) always going to plague me?
What do I do? I want to finish this last year of school as strong as possible, med school or not. How can I change my behavior and my mindset?
I let things like this test grade get me down so much, it turns into a slippery slope (i did horrible on the test so I'll do horrible in the class and I'll never get into med school or anything else).
I hate thinking like that. I hate thinking my life is meaningless. I hate thinking that I don't want to live my life if it's always going to be this hard. I hate thinking that I am not good enough.