I am not myself today

The person you think of as 'yourself' exists only for you, and even you don't know 'you' and all that you are capable of, or what you would do in any future circumstance.  Indeed were you to again become presented with an historical circumstance you might feel and behave differently  'next time'.  If you were able to return to an historical scenario you would find that other people might be re-visiting too...so would have changed since through their original experience too, so they would also behave differently, and therefore surprisingly.

Through experience your preferences and opinions change.  Therefore the 'you' that you feel 'yourself' to be is therefore not a constant...so how could others who know you really know you?

Every person you meet or have a relationship with, or anyone who sees you on the street, creates a version of you in their heads.  To the young you may appear elderly, to an older person you may appear youthful. A person of shorter stature will regard you are being a taller person...your shape and size will differ within the minds of others similarly.  As you spend more time with a person further complex 'who you are' pictures become formed within their minds.  Your mother will know 'you' differently to the way your father knows you.  Friends, co-workers, siblings and neighbours will all form their own version of 'you'.  Each person will only see 'you' from their own perspective and it is their version of 'you' that is held within their minds.

There are a therefore a million versions of 'yourself' held within the minds of others.  A part of 'you' exists in each version. Yet no-one can know the real you...because each person sees you differently and also you change 'yourself' through experience every day, just as they also change themselves. 

As the 'you' that you feel to be 'yourself' to be constantly changes...does the 'you' that is 'yourself' exist...or does the 'you' that is 'yourself' exist only along with the many versions of 'you' that is held within the minds of others...who are themselves constantly changing their minds?