"We are so accustomed to disguise ourselves to others that in the end we become disguised to ourselves."
Francois de La Rochefoucauld (1613 - 1680)

Language is a culprit here as well. We talk with others as well as ourselves by means of language, and many times the sentences we use to describe ourselves to others are also the sentences that we use to describe ourselves to ourselves. If we reveal a particular description of ourselves repeatedly, ultimately our own mind becomes accustomed to that description, and takes it to be true. And it is not just a case of disguisement. Sometimes, our personalities really do change, adapting ourselves to the description we are using.