On my yoga retreat I took a seminar called What is Your Life's Purpose? It was very yoga-ish and touchy feely, but helpful none the less. Before starting on the exercises, we defined "life's purpose" as anything that, when you do it, you feel completely satisfied on a grand scale. Eating ice cream is satisfying, but on a grand scale? Probably not so much. Your purpose might be something you enjoy so much that you lose track of time when you're doing it and/or it makes you feel really proud when you are done with it.
The most useful part was when we had to write for a few minutes to answer the question posed in the course title: What is Your Life's Purpose? Try it. You may be surprised.
I wrote a lot about teaching writing, about showing people how amazing your life can be if you find a way to express all those deep down thoughts. I wrote nothing about getting published in women's magazines, which may be why every time I try to set that as a goal for myself, I don't go after it with as much gusto as I'd expect.
Let me know what you find out!