'Pretty enough' or 'smart enough' for whom?
To whom are you comparing yourself to? I know it may sound trite, but you need to learn to love yourself with your flaws and all. Just as you are, you are whole and complete, and unique in your own way.
If you're not physically fit - then get in shape. If you're not doing well in school, then study harder. Focus on what you want to change, and take action.
I don't encourage self-help through self-delusion, but for most of us, it's a lot easier to criticize and beat ourselves up than to praise ourselves for the things we do well.
Building self-esteem and confidence is akin to building muscles. You need to start off with the lighter weights and build yourself up. Don't get discouraged. If things aren't working for you, try changing things up - but whatever you do, don't give up.
"Persist and you will succeed!"