Whether or not we were created or we happened by chance, we still exist. Whether we are real or not, we still are. Whether there is more or it's all over, in this world or a next, let us just be.

I do to become. I do therefore I choose. I choose therefore I think. I think therefore I can. I can only cause I am and I am so I become...

Purpose is found among a vast sea of many meanings from which each picks that which shall compete the statement above. Influenced or not; Predetermined or not - this selection is still made regardless of belief, precisely because it is belief.

Thus, our existence precedes this purpose but proceeds from a sea of meaningless. For ambiguity, or many meanings, is just a synonym for meaningless.

Embrace this and you shall live! Freedom is but the choice to choose a master. The 'how you will exist' is up to you and for as long as you exist, the only permanence is this capacity for decision not the choice itself you make.

The question thus gradually evolves from "Why do I exist" and "What is my purpose" to "What do I become" and "What should I spend my time doing"

However, be afraid not of a mistake. Simply learn from it for all it needs to be rectified is a different mind from s/he who made it. Though, with that being said, still be careful not to break that which cannot be entirely mended.