That is the simple secret of happiness.
Whatever you are doing, don’t let past move your mind; don’t let 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.