"If you look to the world, you'll be distressed. If you look within, you'll be depressed. If you look to God, you'll be at rest." -Corrie ten Boom
I'm struggling with that right now, obviously. I have depression myself. But this is the problem with looking to yourself for a purpose. You're a fallen person, as all people are. You need a savior, and humans are built to know that they do. Even if you don't admit it to yourself, you'll figure it out subconsciously, and you will be unhappy.
I hope you find your purpose, friend.