"Fake it 'til you make it" (also called "act as if") is a common catchphrase. The purpose is to avoid getting caught in a self-fulfilling prophecy related to one's fear of not being confident.
In some cases it may be recommended as a therapy technique for combating depression. In this case, the idea is to go through the routines of life as if one were enjoying them, despite the fact that initially it feels forced, and continue doing this until the happiness becomes real. This is an example of a positive feedback loop.
The phrase is often associated with Alcoholics Anonymous even though it does not appear in either of the books that form the foundation of the AA program, Alcoholics Anonymous or The Twelve Steps and Twelve Traditions. AA to Z; An Addictionary of the 12-Step Culture describes it as a "suggestion often made to newcomers who feel they can't get the program and will go back to old behavior. The suggestion implies that if the newcomer acts according to the steps and teachings of the program, then the program will begin to work and the anxiety will fall away".
It does not always lead to success. For example, see cargo cult science.