Life's purpose

So, as senior year is starting to fly by and college apps. open in about 14 days, I'm beginning to ask myself, why? Why are we here? I mean, I know we're here for the sole purpose of serving God, and for glorifying Him in all we do. But does that mean that it doesn't really matter what profession we go into? Obviously we're not going to do any of the off the wall stuff to serve God, but does it really matter? And how do we know we're doing the right thing? How do we distinguish God's voice from our own? Essentially, in the end, I think that none of it really matters. In Luke 12:7 it says "And the very hairs on your head are numbered. So don't be afraid; you are more valuable to God than a whole floch of sparrows." (NLT) So, should I take that to mean that it really doesn't matter what we do with our profession, our life, as long as we're following God?

And then that same question just keeps creeping up; How do we distinguish between God's voice and our own heart's desires?