Gerere se aliquo modo is an idiomatic expression that according to L&S means: "to bear, deport, behave, or conduct one's self, to act in any manner". I think this makes very good option for what we are seeking, since the imperative "Be" here is really in the sense of "act as".
aliquo modo might be replaced with ablative (as in the formula), but also with an adverb ("turpissimē"), but even with simple adj. ("se medium gerere" - to remain neutral, Liv. 2, 27, 3.). So we might think of some different options here:
- Gere te liberē (liberē - an adverb that means freely/unrestrained)
- Gere te naturalem/naturaliter (naturalem - adjective here; naturaliter - adv.)
Eventually, however, had I to chose one, I could not have resisted of using a quite cool word that I have recently learned (The hearer of which is likely to never forget it):
tuātim (=in your way/ tuo more) [we also have meātim]. It produces:
Gere te tuatim.
I like the repeat of tu here, I think it adds to the message. But as tuatim is very rare word, one might be inclined to replace it with tuo more: