We all have "things" we identify ourselves by, even if we don't actually walk around with those labels displayed for everyone. You may call yourself a smoker, a health nut, a fat person, a stupid person, an intelligent person, etc.
It's interesting to look at how our personal conceptions of our identities inform how we can actually (or can't) change. I think the easiest example is to look at someone addicted to something---let's make it obvious and just talk about heroin :)
So a person who calls themselves a "heroin addict" is never going to be successful at changing (most likely). Why? Because if you see yourself as a heroin addict, you may be able to stop using it temporarily, but you're still an addict who is not using it and you will start again. This is really common for people who are overweight too---if you call yourself a chubby person, you will have a much more difficult time changing it permanently (ever heard of yo-yo dieting?).
Let's say you see yourself as a health nut. If you label yourself as "health nut" and really read up on it and believe it to be true and start acting like a health nut, you can change your weight. Eventually, the habits you do that are causing your extra pounds will change. But if you focus on being overweight, you are just fulfilling the label.
There have been so many studies on this, but we still don't really follow it (maybe we like to call ourselves names?). A related example is a study in which classrooms were labeled as either regular or gifted (where there were no difference in the abilities of the students, just randomly selected). The "gifted" students not only thought of themselves as smarter but they also performed as gifted students would. Even those students who had been previously labeled as needing extra help. This study was used to demonstrate that the way teachers perceive their students determined how they taught and treated the students. They held students to higher standards and the students themselves also knew they were being called gifted, so they lived up to the expectations.
Why not use this to our own advantage? What do you want to be? Smart, organized, beautiful, funny, life of the party.........? If so, then stop calling yourself average or worse. Make lists of the identity traits you want. Then focus on those instead of the names you've been calling yourself up till now. Just see what happens!