Not sure if it was the very first or not, but it’s the one I remember from junior high. Not necessarily a poem, but poetic enough, and memorable enough since it’s from a classic.
The Litany Against Fear from Dune by Frank Herbert
I must not fear.
Fear is the mind-killer.
Fear is the little-death that brings total obliteration.
I will face my fear.
I will permit it to pass over me and through me.
And when it has gone past
I will turn the inner eye to see its path.
Where the fear has gone there will be nothing.
Only I will remain.