Why is being yourself so hard?

Having been there it's two things: first, if you adopt a personality and get rejected it's not you that got rejected it's your personality. If you have low confidence that's an insulating layer you feel you need. Second, a lot of people go through a transition from childhood to being a teenager where everyone starts acting differently. Personally, two weeks into 6th grade my best friend from 5th grade answered the phone with, "Stop talking to me. I hate you." Not a lot of children have the awareness or ability to act differently from themselves so if they go through only rejection at that transition, they'll become afraid to show themselves and that fear takes a long time and a lot of effort to unlearn.