(I know I said I was signing off, but got caught in a bit of a rabbit hole!)
It's part of a longer poem by Grace Noll Crowell:
*This, too, will pass.
O heart, say it over and over,
Out of your deepest sorrow,
out of your deepest grief,
No hurt can last forever-
Perhaps tomorrow will bring relief.
This, too, will pass.
It will spend itself-
Its fury will die as the wind dies down
with the setting sun;
Assuaged and calm, you will rest again,
Forgetting a thing that is done.
Repeat it again and again,
O heart, for your comfort;
This, too, will pass
as surely as passed before
The old forgotten pain, and the other sorrows
That once you bore.
As certain as stars at night,
or dawn after darkness,
Inherent as the lift of the blowing grass,
Whatever your despair or your frustration-
This, too, will pass*