Spend time helping others for a while.
This is actually a very selfish thing to do, no matter how counter-intuitive that sounds.
Studies on human happiness have shown repeatedly that we are actually made happier by giving to others than by receiving things.
It doesn't really matter what you choose.
Build houses with Habitat for Humanity,
Try Volunteers for Peace and dig wells and build homes in the third world. http://www.vfp.org/
Try volunteering to help children with Unicef.
You'll find that physical work without the burden of worrying about things like "what's it all about" or "what's my passion" will make you happier, give you great experiences and memories, and introduce you to things about the world that could positively affect you for the rest of your life.
Forget startups, you're in a position to do what few people have the opportunity to do.