There are only two things you must do to be happy. The first is let go.
Let go of mistakes of yesterday; even if you had lived perfectly happiness is not found in the past. Also let go of tomorrow, the torments you build yourself are worse than tomorrow’s troubles, should they even arrive.
Once you have mastered letting go you are ready for the next step. There are only two things you must do to be happy, and no one knows what the second one is.