Existiental dilema?

Ok that's the best thing I can think of to describe what I'm going through at the moment. I seem to feel like I have little reason here for existing. Not saying I want to end my existance or anything, that's out of the question. But right now I just feel like I have little reason for being here. It's not even that though, I just feel like my life really doesn't hold that much value and that I don't feel capable of going places. I've been having this nightmare ever since I entered highschool. Don't know if it's just a fear or if it actually speaks the truth.

You see, I'm the kid who never does well in classes even though I'm plenty intelligent enough. I feel terrible about this everyday because I could be getting high nineties and acing school. I just have so much trouble with it and it is hell for me because I have a learning disability. I've never been good at school although IQ tests reveal that I'm near being gifted. It just doesn't make sense. I feel like a complete failure because I can't get above a 65 in most of my classes. I have no motivation whatsoever to do homework. Every time I sit down to it, I get overwhelmingly anxious and think that I'll never get it done. This anxiety is so bad that I usually end up having to abandon the work because I'm too worked up to even look at it. I've denied this problem for so many years and now it's so bad that there seems to be no hope in fixing it.

It's horrible because I have a little brother who really looks up to me and sees me as one of the most important people in his life. How can I ever be a good role model if I can't even do good in school? I also have younger friends to set a good example for. I want them to do good in school and stay out of trouble, but how can I say that when I can't even do it myself?