Peers, teachers, colleagues and even parents all attempt to define you in different ways. Most people keep giving you labels for as long as you come into contact with them. These labels serve as reminders and many times they effectively amount to negative reinforcement - nobody wants to be known as the "fat dude," for instance. Being referred to as tall or well built on the other hand is not as hurtful as being called short or skinny, because society has different connotations of each of these terms.
What 99.99% of us do is we collate all these labels, opinions, comments and assessments made by outsiders to form our own self image and perception of ourselves. This perception then projects into our actual lives and permeates our decisions. A child who is constantly told that they are weak or too skinny will in all likelihood never dare to seriously pursue any sport as they grow up, for instance. There are always exceptions, but by and large this is the reality: we tend to fit into the mould that others create for us. Taking the careless, insensitive labels given to you by others to heart will cause you to live life in a very confined little mould.
However you can't be on guard all the time and fight every dis-empowering, belittling or insulting label given to you. Excessive vigilance in itself will eat you up. What works best is to simply ignore both insults and praise. In both cases they are usually misjudgments of you anyway. Some say you can choose to ignore the hurtful labels and give attention to the empowering ones that people give you; but the fundamental error in this is that you are still leaving yourself susceptible to the words of others. An insult can follow 5 minutes after the same person gave you lavish praise. Why leave your self image and happiness susceptible to the fickle opinions of such people?
Even positive reinforcement can confine you. Someone can be heralded for their great mathematical ability with comments like "she's gifted with numbers" or "she's extremely sharp at Math." The hypothetical girl in question may be keen on marine biology and in fact have little interest in Mathematics, but chances are good she will end up choosing a mathematics related career because constant reinforcement of this ability can lead her to close off other possibilities. She risks growing up thinking that Mathematics is her calling because of the wide acclaim she receives for it. For this reason it is best to ignore both the praise as well as the insults. Acknowledge the praise but don't let it affect your opinion of yourself because taking praise too seriously often causes you to make decisions based on earning more of the same praise.
You are not your race, sex, age, size, current ability, name, title or even education. You just are. Any label beyond your name is a limitation, a stereotype. The greatest tragedy is that people let these labels cross over from opinion to truth and once this happens their path is already laid out for them. Keep yourself open to the vibrant unpredictability and infinite possibility that life has to offer. Stay undefined.