Just believe in yourself. Even if you don't, pretend that you do and, at some point, you will.       

         ~Venus William

Meaning: What does believing in yourself mean? It means to have faith and belief in yourself that what you do is correct. If you have faith in what you do you have believe on yourself. 

If you have believe in yourself you can do any task that is given to you.For example take Ivan Origone, the fastest skier in the world. At first nobody supported his skiing. Everybody thought that he was just wasting his time doing skiing. He never listened to anybody, he just believed in himself and started practicing more and more. Eventually he came out to be the fastest skier in the world. How was it possible? It was possible just because he believed in himself. He told himself that he can do it and he really succeeded in doing it. 

 You may not realize it or you may not want to believe the fact that what you did is correct. Just try to pretend believing in you. If you start pretending you feel like believing in yourself and over time you really start believing yourself. That is why it's said to have believe in oneself. So believe yourself and proceed. Everything will turn out well one day.