We are constantly in the process of creating ourselves. A person's identity is not a fixed entity, although we often treat it that way. Identity is dynamic and frequently changes over time. (I, for one, am definitely not the same person today that I was, say, in high school). An ever-evolving identity means that we are never "stuck" being some way we'd prefer not to be. If there is some way that we want to be - confident, assertive, social friendly - we can become that way.
This is where the technique of "acting as if" comes in. A person can act "as if" he already possesses the quality he wants to embody. The more often he does this, the more comfortable he becomes exhibiting the desired quality. People will respond to him as if he already possesses the characteristic in question. Eventually, the person will find that he no longer has to "pretend;" he will discover that at some point he actually became the way he wanted to be.
People are, at times, resistant to approaching change in this manner. They argue that it would be "fake" or "phony" to behave in a way that is inconsistent with their emotions. I encourage those who make this argument to take a look at what is driving these emotions. Are they based on unreasonable beliefs that were developed early in life and never re-examined? Take, for example, a person who wants to be confident. She does not feel confident and so believes it would be "fake" of her to pretend to be confident. But why does she lack confidence in the first place? Is it rooted in unreasonable beliefs such as, "I have to be perfect for people to like me" or "My feelings don't matter?" Does it make any sense to allow our behavior to be dictated by emotions that are based on unreasonable beliefs? Wouldn't that, in fact, cause us to behave in unreasonable ways?
The fact is, the way we define ourselves is constantly changing, so long as we don't cling to any one particular set of ideas about who we are. Our feelings and beliefs will at times determine our behavior, and rightly so. This should, however, be a conscious choice, not blind obedience. If we decide our beliefs about a particular thing are unreasonable and that our emotions about it are unhelpful, we can choose to set them aside. In this way, we avoid placing limits on who we can become.