[1] It was published (posthumously) in 1848 in Life, Letters, and Literary Remains, of John Keats by Richard Monckton Milnes.
[2] When I have fears that I may cease to be Before my pen has glean'd my teeming brain, Before high-piled books, in charact'ry, Hold like rich garners the full-ripen'd grain; When I behold, upon the night's starr'd face, Huge cloudy symbols of a high romance, And think that I may never live to trace Their shadows, with the magic hand of chance; And when I feel, fair creature of an hour!
[4] He fears he will no longer be able to write, witness the beauty of the world, or experience love or fame once he dies.
[5] The shore and water that love and fame sink within represent an expanse of fears that sit before Keats, giving the natural world a darker theme in those lines.
Thoughts are tangible items to be grown into "high-piled books," as Keats feels he can allow his ideas to flourish if he only had a long enough life.
Stars, cloudy symbols, shadows, reflect the intangible beauty of the world which Keats can also not attempt to understand because of a life cut short.
[6] The couplet shows abstract concepts of Love and Fame becoming tangible, though they sink to nothingness as Keats realizes he has no time to achieve them.
[6] The first four lines express Keats' fear that he will die before he has written all the works he hopes to, "before [his] pen has glean'd [his] teeming brain.
Keats is condemned to a short life by chance, and because of that he will remain unable to trace or understand how fate functions.
[12] The last three lines of the poem which describe "the shore" and state, "Till Love and Fame to nothingness do sink" may relate to the reference to water in Keats' epitaph.