Commentary on Meditations: B5:23

Reflect often on the speed with which all things in being, or coming into being, are carried past and swept away. Existence is like a river in ceaseless flow, its actions a constant succession of change, its causes innumerable in their variety: scarcely anything stands still, even what is most immediate. Reflect too on the yawning gulf of past and future time, in which all things vanish. So in all this it must be folly for anyone to be puffed with ambition, racked in struggle, or indignant at his lot - as if this was anything lasting or likely to trouble him for long.

Change is the one constant in the Universe.  What a folly it is, then, to hope for things to remain unchanged.  What a folly it is to hope that "this one moment" might last forever.  What folly to yearn for the past or hope for the future.  And vice versa - what folly to disdain and hate the present moment because you're in pain.  What folly to wish for change now - because it will come anyway.

The lesson here is to embrace the change - love it!  Seize the day (now)!  Don't seize tomorrow or yearn for yesterday.  Focus on the now; focus on what you can do now.  Sure, plan for the future, but not too much at the expense of now.

To help you embrace the now, reflect often on how life and everything is like a river.  It flows and moves endlessly.  Time is the same - it will always move along.  Therefore, embrace now - this minute, this hour, this day.  What happens now is sufficient for your attention.  Don't let your focus spill over into yesterday or tomorrow.

(see also Citadel p. 118, 253)