Your purpose in life is chosen by God. It is not negotiable. God had a plan in mind when He allowed your spirit, which was in heaven, to come into flesh and blood and be born on earth. There is no changing God's purpose for your life, but you will not necessarily fulfill it because free will involved in that process.
God tells us that He has even decided when we would live on earth and where we would live.
The Bible says, "the gifts and the calling of God are irrevocable" (Romans 11:29). That is why someone can be a successful preacher and fall into sin and still have fruit from their preaching. Our gifts and calling are irrevocable. However, God will eventually deal with the sin of anyone who continues to misrepresent Him.
Oswald Chambers explains another dimension of calling in his September 29th Utmost for His Highest devotional:
We are inclined to forget the deeply spiritual and supernatural touch of God. If you are able to tell exactly where you were when you received the call of God and can explain all about it, I question whether you have truly been called. The call of God does not come like that; it is much more supernatural. The realization of the call in a person's life may come like a clap of thunder or it may dawn gradually. But however quickly or slowly this awareness comes, it is always accompanied with an undercurrent of the supernatural—something that is inexpressible and produces a “glow." At any moment the sudden awareness of this incalculable, supernatural, surprising call that has taken hold of your life may break through—"I chose you . . (John 15:16). The call of God has nothing to do with salvation and sanctification. You are not called to preach the gospel because you are sanctified; the call to preach the gospel is infinitely different. Paul describes it as a compulsion that was placed upon him.
If you have ignored the great supernatural call of God in your life, take a review of your circumstances. See where you have put your own ideas of service or particular abilities ahead of the call of God.
Paul said,“... woe is me if I do not preach the gospel!" He had become aware of the call of God, and his compulsion to “preach the gospel" was so strong that nothing else was any longer even a competitor for his strength.
If a man or woman is called of God, it doesn't matter how difficult the circumstances may be. God orchestrates every force at work for His purpose in the end. If you will agree with God's purpose, He will bring not only your conscious level but also all the deeper levels of your life, which you yourself cannot reach, into perfect harmony.