Developing the ability to be able to stand up and express a Perspective that is contrary to the prevailing "Supposed to be;" is one of the major values in all of this.
Start by undoing the underlying assumptions typically inherent in that question. Usually the assumption is that there is One "Life's Purpose." In addition, it is assumed that one's Life Purpose is "out there somewhere." Something predetermined.
Self-Belief doesn't mean you see yourself being as perfect as you are but rather seeing yourself as what you'll become.
You'll be very productive at whatever you do when you pay keen attention and do not lose focus. How well you focus on the task you have at hand is what'll determine your level of success.