Certainty of what makes us human is certainty of the end of our existence. The only true certain event in life is death, and this inevitability is rightfully intimidating. It is not only healthy to have a fear of death, but it is necessary to the survival of our species. The survival instinct is a powerful motivator, helping us to avoid an untimely demise and maximise our contribution to our species (which need not necessarily be procreation). This fear reaction does not incite something equally important to our global survival and to our individual lives: having a great existence.
We aren't all going to be Isaac Newton, Charles Babbage, or Steve Jobs. Having a great life is not about being financially successful, making some grand contribution to the wealth of human knowledge or radically changing the world. Having a great life is about being able to do the things that make you happy, being able to spend time with people that are happy to have you around, and finding a way to leave your mark on society in whatever small way you can achieve.
Leaving your mark on society should not seem like a grandiose requirement. You really can't help yourself, unless you truly are a reclusive hermit. When you work, you pay taxes, you contribute to the wellbeing of society. When you help someone, your support enables them to do more than they would otherwise. If and when you find something more direct, or more satisfying that you can do, then you owe it to yourself to make the most of that opportunity.