Do not "Believe in youself". I hear this all the time, so I'm sure you're going to be hearing this too.
But I haven't found anywhere in the bible where God says to "believe in yourself."
When the world the tells you to "believe in yourself", think about the words in Proverbs 3:5-7;
5 Trust in the Lord with all your heart,
and do not lean on your own understanding.
6 In all your ways acknowledge him,
and he will make straight your paths.
7 Be not wise in your own eyes;
fear the Lord, and turn away from evil.
Also, David didn't defeat Goliath by believing in himself. Others doubted David, but:
David said, “The LORD who delivered me from the paw of the lion and from the paw of the bear will deliver me from the hand of this Philistine.” And Saul said to David, “Go, and the LORD be with you!”
(1 Samuel 17:37 ESV)
Put your faith and trust in God.