Responsibility
On the one hand we are responsible for all things in our own life. On the other hand we are responsible for nothing. Case in point: you live your life and adjust to all circumstances that appear before you. You are responsible for your responses. They define you and they form the foundation for all that is to come. You are fully in charge of your responses and thus fully responsible for them.
But you have no control over what is to come. You have no say in what tomorrow may bring, nor indeed if there will be a tomorrow. In this manner you bear no responsibility. They key to wisdom, happiness, and success in life is to balance responsibility for one's responses against the utter lack of responsibility for one's environment. One must see where the one ends and the other begins.
This is not easy. Many confuse the two and attempt to bear the burden for their environment - a thing they can not do - while ignoring their own responses to their environment - a thing they must not do. This ends in confusion, pain, chaos and suffering.