The Sickness Unto Death by Sören Kierkegaard

Chapter 3 The Forms of This Sickness, i.e. of Despair

In every instant a self exists and is in the process of becoming. The self does not actually “exist,” but is only that which it is to become. In so far as the self does not become itself, it is not its own self, and not to be one’s own self is despair.

From: http://www.religion-online.org/book/the-sickness-unto-death/