Perhaps you are lucky enough to have a clear winner emerge in your choice of Life Purpose. Something that excites you more than anything else. Something you cannot imagine a day without. My hat is off to you. You will achieve greatness on this earth.
For others of us the choice is not so clear. We have several candidates, all dear to us and with no clear winner. For me, for many years I considered that to be a permanent situation. And that I would never be able to come to a clear choice, or that if I did make a choice I would be wracked with doubt. I have, however found two techniques for making a choice: one magic and one scientific.
The magical deciding technique is the ancient i-ching. I will write more about how to use the i-ching and share stories of its uncanny ability to cut to the core of a decision.
The other technique is borrowed from the web development plan at Site Build It. Dr Ken Evoy calls it The Chooser. I like to call my, slightly modified version, Decide™. The idea is that you write down values and outcomes that are important to you like: make money, enjoy my work, create something of beauty, fame, time with my family. You then assign a rating to each one of those based on their comparative importance to you. You then take each Life Purpose Candidate and rate how much it contributes to each of your values. You can then calculate a winner. If you don't like the answer you can ask yourself why and thus discover another value that is important to you and re-calculate. It is powerful and works for nearly any type of decision.
I can provide a spreadsheet for you so you can try it yourself. I'm also considering putting a web-version of the Decider on this site. Please let me know if you are interested in this idea.
All that said, there is another option: Refuse To Choose.