On a simplistic physical level, you are 1.) Your genes and 2.) How those genes express themselves according to the environment. 1. is a fixed variable; 2. is infinitely variable and can be manipulated (even by your own actions--e.g. lifting weights/winning competition increases testosterone production, makes changes within the lymbic system, changes behavior).
The environment around you is constantly in flux. So, for example, you'd be a very different 'you' with a gun pointed at your face vs. who you'd be in front of a crowd chanting your name. Or imagine you were super blessed genetically but abused as a child. Are 'you' those good genes or are you how that genetic information reacted to that abuse?
Then you throw in social conditioning and things get even more obscured. Are 'you' the you who gets horny and goes and looks at some really twisted p0rn or are 'you' the you who feels a little bit of shame afterward? Are 'you' the you who really, really wants to fvck that hot brunette across the room or are 'you' the you that doesn't want to bother her and chickens out. Which drive is the most legitimate expression of 'you?'
Getting to the bottom of those problems requires more conscious mental effort than you'd be able to do while simultaneously functioning in day-to-day life. So your brain simplifies things and makes assumptions based on imperfect information. Thus, the ego makes all sorts of rules and labels of who 'you' are to rationalize that behavior. Oh, I didn't approach that hot brunette--it wasn't because I'm chickensh1t, it's because I'm a respectful guy. I'm just an introvert. That's not me. Etc., etc.
Those 'identities' can be adaptive or maladaptive depending on your environment and how your identity allows you to function within that environment--and whether you are able to achieve the outcomes you want. So what's the good of 'being yourself' if that version of 'you' is 90% bullsh1t rationalizations and doesn't help you achieve what you want to achieve? On some level you do have a core identity (genetic limitations on your range of behavior), but the only way to get into alignment with that core is to constantly test your comfort zone and your assumptions in a wide range of environments to find out what really works for you--and even that'll change over time.