This seems a ridiculous thing to say ...who else could you possibly be? Everyone else is already taken.
But of course it is all about: not being a copy or parroting someone else, trying to please everyone, or wearing a mask in order to be in the world.
This is not accepting or loving yourself. It is dancing to someone else's tune, or avoiding life.
When using the expression, 'be yourself', this is what is required: Accepting, Valuing, Forgiving, Blessing, Expressing, Trusting, Loving and Empowering YOURSELF!
It is putting yourself first, making sure that your feelings are considered and respected. You teach people how to treat you, so you need to make sure that you are honouring yourself in order for others to do the same.
You can always tell when someone cares about themselves, they stand tall and exude confidence. They have accepted themselves, just as they are, warts and all.
It's a process, it takes time to get to the bottom of all the hurts, anger and resentment we carry with us, but it is worth the work. To try and find out why you live in survival mode and are not truly happy will be the greatest work you will ever do, and have the best results. You are worth it.
"If you make friends with yourself you'll never be alone." Maxwell Maltz