Keats is afraid of a number of things. First of all, he's afraid that he will die before he's had a chance to fulfill his creative potential as an artist. Keats uses an agricultural metaphor—"full ripened grain"—to refer to the great books which he still hopes to write, and in which he hopes to store his ideas, the vast content of his mind, before he passes away.
As he gazes up in wonder at the night sky, Keats is also afraid that he will no longer be able to conjure up great works of art out of nature using his poetic skills—"the magic hand of chance," as he calls them.
Finally, Keats fears that he will never again be able to look upon that "fair creature"—the woman that he loves—and that he will never experience the magic, the "faery power" of spontaneous love. Whenever Keats becomes afraid of what he might lose before his time is up, he focuses his attention instead upon the end of mortal time until all fears about the loss of earthly fame and love have dissipated to nothingness.