TO BE thine own self, or NOT TO BE thine own self

POP1

In Shakespeare's Hamlet, Polonius says:

                  "...... to thine own self be true,
And it must follow, as the night the day,
Thou canst not then be false to any man."

Polonius gives this immortal advice to his son Laertes who is leaving home to begin his university education.

POP2

Here's another quote, author unknown:

"I've been many things in my life, but the one thing I've never been is myself."

Reconciling these two quotations could mean the start of a new life.

POP3

Given the character of Polonius that Shakespeare was fleshing out, what his advice probably means is, "look out for yourself first; when you take care of yourself, you'll be in the best position to take care of others." In this case "true" would mean "loyal to your own best interests".  We shouldn't mistake this for Shakespeare's so-called "universal wisdom".  He was a writer of fiction after all.  It does make sense though; if you want to be charitable, you need to have the money to give.

Perhaps it's best to ignore this bit of realism.  Let's focus on the poetic sensibility of the lines and stick with a meaning where the poetry speaks to the heart.