I can't do well in Algebra (or substitute in whatever course you're worried about).
I can't make the team.
I can't get that part in the theater production.And, you know what? Sometimes you're right. You're not always going to be successful, and certainly not always on the first attempt. Failure is always an option and, in fact, almost everything that's worthwhile achieving involves lots and lots of failure along the way.
But what happens to many of us is that we guarantee that we'll fail - and fail permanently - by not even trying. We convince ourselves that we aren't capable. We convince ourselves that it's better not to try than to try and perhaps fail in public. We convince ourselves that it's just not possible.
I want you to rethink what's possible. I'm sure many of you watched Oscar Pistorious at the Olympics this past summer. Here's a link to a video from the Olympics, and here's a picture from the ParaOlympics where he set a ParaOlympics world record in the 200 m (source).
How easy would it have been for Oscar to not try? How many times has he "failed" in the traditional sense of the word along the way?
Yet how amazing what he has achieved. Don't you think you can learn something from Oscar?