'You have to believe in yourself'.
It's great if you believe in yourself... But still to me that sounds as an 'in between station' towards the real goal: BE YOURSELF.
Because when you are yourself, there is nothing to debate, to discuss or to believe. You don't have to convince yourself or anyone.
When you ARE YOU... you know instead of believe.
So my advice is: Don't believe in yourself, but BE YOURSELF and you will know.