We are all imposters to some degree. Survival in this world essentially requires that we pretend to be someone we're not - and not only one someone - we have to be a different someone to suit different people and occasions. It's really no wonder that everyone is looking for themselves, we're all lost under the rubble of our own bullshit.
Can you honestly say that you are always yourself, 100 % of the time? Can you honestly say that you have never changed your stated opinion to suit the moment (ie to fit in)? Can you honestly say that you have never held yourself back for fear of humiliation or ridicule? Can you honestly say that you have never felt the need to adjust your appearance or personality to suit the career, goal, partner or social status you aspire toward?
Who are you with your family?
Who are you at work?
Who are you in school?
Who are you with your friends?
Who are you with your partner?
Who are you alone?
Who are you with an authority figure?
Who are you when you are in the position of authority?
Who are you in nature?
Who are you with animals?
Who are you on the internet?
Who are you when there's conflict?
Who are you with people you respect?
Are constant, or do you adapt to fit in? What are the reasons for you changing yourself? Do you do it because you see that as the best way to direct the situation or because of some fear within you? Do you change because you believe that you must be a certain way in order to fit in or get what you want? It's not about always being honest with everyone all the time - that is not what I am advocating. What I am opening is the consideration of investigating WHY you change - is it because you have made a clear and deliberate choice to do so, or because you are functioning according to some belief, fear, desire, ideas or whatever?
When you are in a moment where you know that your actions and words are not what you really want them to be, and the only reason you are being dishonest is because you're following some twisted, fuzzy logic about what you 'should do' or about what people will think of you if you say and act according to your true 'inner voice' - stop for a moment. Take some time later on in the day to really investigate why you believe you should speak and act in this way. Write about that moment, write out your specific thoughts and rationalisations (all the reasons you gave yourself for fitting in). Those thoughts will tell you much about the source of your insecurities etc. This simple exercise may surprise you - I dare you to give it a try.