What are some poetic devices in Keats's "When I Have Fears"?

The poem “When I Fear That I May Cease to Be” by John Keats contains many poetic devices, the most obvious of which is the Shakespearean Sonnet form. This means the poem follows the end rhyme scheme ababcdcdefefgg. Like all Shakespearean sonnets, the poem also imitates a heartbeat with iambic pentameter, making the sound of a heartbeat for ten syllables in each line: bah-BUM bah-BUM bah-BUM ba-BUM ba-BUM. Iambic Pentameter, which often is used for subjects of high passion or drama, is well selected for the subject of this poem: fear of dying before success and the fulfillment of love.
Keats died just a few years after writing this poem, and when he wrote it, he considered his talent as a writer to have reached its height. The agricultural reference in the first quatrain of the poem uses an extended metaphor to compare his creative writing to a harvest that has not fully ripened.

When I have fears that I may cease to be
Before my pen has gleaned my teeming brain,
Before high-pilèd books, in charactery,
Hold like rich garners the full ripened grain;

Besides metaphor, the poem also uses assonance to help the reader feel his sense of urgency: “Before my pen has gleaned my teeming brain” he says. It almost sounds like a heaving plea. “High Piled also creates a musical effect with the “I” sound in that term, which almost sounds like a heavy sigh. Alliteration—the repetition of consonant sounds—is also present in the poem, which lends to the overarching musical effect. The most obvious is “feel, fair” in the third quatrain. At this point in the poem, Keats is reflecting upon how he will never get to see his love interest after his death, which makes his grief for unfulfilled fortune seem trivial. With all the musical devices, it is almost a dirge.

And when I feel, fair creature of an hour,
That I shall never look upon thee more,
Never have relish in the faery power
Of unreflecting love—then on the shore
Of the wide world I stand alone, and think
Till love and fame to nothingness do sink.

This last part of the poem again uses metaphor. His love interest is called a “fair creature of an hour,” or a beautiful and temporal entity. He grieves the passing of beauty in this line, as well as his own death, which will take him from her prematurely. In lines 10 and 11, he compares love to a “faery power / of unreflecting love,” which means he feels love has power over him; he is overcome by the strong passion and emotions of his romantic feelings, and so much so that reason abandons him. This is how strongly he feels about his love—that reason has left him. It just makes dying that much more tragic, which is why in the couplet, he grieves the isolation that comes from early death with metaphor. The world becomes a “shore” that he “stand[s] alone” on, thinking of the magnitude of his losses.