At one point in time, we will all experience something that makes us feel like we aren't good enough. That is, regardless of whether we have tried our best or not. In times like these, all you can do is tell yourself that you have done all you can, and that "you are exactly where you should be... doing exactly what you should be doing."
Don't ever let anyone tell you that you aren't good enough (whether it is directly or indirectly), because if you have the passion, motivation, talent, and perseverance, you will get to where you want to be.
My BFF, R, gave me this card around Christmastime. Whenever I'm feeling discouraged about my future (as I had been this week), I pull this out of my desk, and read it several times.