Taken literally, this "Be yourself" command just asks you to continue your miserable existence unaltered. Come to think of it, you have no choice but to be yourself (unless you are considering some form of black magic to convert to someone else).
The more natural explanation is "be your usual lively self", which for most people doesn't work. The better advice at this point would be "to fake as much as you can be to pretend to be that awesome-self". Ironically, the "Be yourself" camp cannot argue against the seemingly opposite advice because your natural self could be one that always try to fake to be a better self.
A more useful advice would be "to pretend to being yourself", which just says that pretend all you want, but be careful in showing that to be your natural self so that people think you are being yourself. That I think will work in all situations.
P.S: I have not left my house in the last 50+ hours. Just a disclaimer on why this entire blog should be taken with a pinch of salt.