This text comes from The Poetical Works of John Keats, a historical songbook transcribed on Wikisource and reproduced from the pinned revision linked below.
Nature withheld Cassandra in the skies,
For more adornment, a full thousand years;
She took their cream of Beauty, fairest dies,
And shaped and tinted her above all Peers:
Meanwhile Love kept her dearly with his wings,
And underneath their shadow filled her eyes
With such a richness that the cloudy Kings
Of high Olympus uttered slavish sighs.
When from the Heavens I saw her first descend,
My heart took fire, and only burning pains,
They were my pleasures—they my Life's sad end;
Love poured her beauty into my warm veins.