Keats here uses very successfully the form of the Shakespearean Sonnet in a beautiful poem which was composed in a competition with Shelley and Leigh Hunt (editor and writer of The Examiner, a radical weekly publication started in 1808).
Particularly poignant is that here he contemplates, not the human agony generally, but personally the inevitability of his own death.
He sees himself as poet, unable at last to realise his thoughts and images in the permanent form of "high piled books, in charactry".
The poem employs the conditional thought structure of Shakespeare's Sonnet 12, "When...when...then", delaying and leading to the final resonantly powerful image of individual loneliness and universal nothingness.