Every now and then, you should be WHERE YOU ARE, WHEN YOU ARE THERE. Not somewhere else. Not worrying about the future. Not reliving the past.
(1) Imagine that time has "locked" for a minute or two, as if you had a Pause button for the whole universe. Then (2) imagine that you are in a sphere, about ten feet in diameter, that contains all of existence; there is NOTHING outside it. There is no "time" or, to put it another way, all time is NOW. BE THERE, right there, in that place, in that time, for a few minutes, not allowing anything to invade your serenity.