Importantly, the origin of ones purpose comes from within each of us. The idea that one absolute guiding light purpose exists can be appealing, alas it is also simplistic, and all too often at the core of a profit making scheme. Sure external factors and third party input should be integrated into one's purpose after due consideration; however they should always be taken with a shaker full of salt. Too often even successful ideologies suffer from absolutist positions, be they religious, socio-economic or political.
Even my heartfelt philosophy of Goodness First has an aspect of ideological absolutism to it. Yet, in spite of the first rule of Goodness First being its very name, the equally implied corollary is the recursive idea of Consider Second. Consider Second simply indicates the necessity of periodically taking a fresh look at all factors involved with an open heart and reasonable skepticism in play.
Of course, once you've recalibrated your purpose, implementation is critical. Critical enough to warrant a dedicated post of its own.