“The mind weighs and measures but it is the spirit that reaches the heart of life and embraces the secret; and the seed of the spirit is deathless.
“The wind may blow and then cease and the sea shall swell and then weary, but the heart of life is a sphere quiet and serene, and the star that shines therein is fixed for evermore.”
~ Kahlil Gibran, Jesus, the Son of Man
You know the eternal by the calmness and certainty it exudes. Everything else is just temporary. Trust that the peak will follow the trough as surely as the trough will follow the peak and know there will always be another cycle.
But as for the eternal, my friend, you’ll find it in your heart when the fluctuations of the mind have stilled.