“Does the Bible teach that you should ‘be yourself’?”
I can think of at least one example in scripture of a person who was better off being himself than trying to be something he was not. Saul clothed David with his own armor to fight Goliath, but David was unwilling to rely on protection he had never personally tested in battle. He left the best armor in the kingdom behind and used a sling and stones instead of a sword.
We all know the results of that encounter. David being David got the job done.