That is the simple secret of happiness. Whatever you are doing, don’t let the past move your mind; don’t let the future disturb you. Because the past is no more, and the future is not yet. To live in the memories, to live in the imagination is to live in the non-existential. And when you are living in the non-existential, you are missing that which is existential. Naturally, you will be miserable, because you will miss your whole life.