In this sonnet, Keats is contemplating the things that inspire him to create his poetry, his art. Because of his fear that an early death awaits him, he expresses his regret at not having the opportunity to fully draw on Nature, "the cloudy symbols of a high romance" he is able to see in "the night's starr'd face." But the "fair creature of an hour" would appear to mean a woman—either women overall or the particular woman with whom Keats was in love, Fanny Brawne.
When he speaks of relishing in "the faery power of unreflecting love," the terminology brings to mind his "La Belle Dame sans Merci" as well. Keats views women partly with what would now be considered a male chauvinist orientation. He often sees women as temptresses (in "La Belle Dame") or as unreachable objects of desire. In the sonnet "I cry your mercy," he pathetically calls out to this same woman-figure he despairs of being united with.
These evaluations need to be placed in the context of the poet's biography. Keats was a physically sick man, suffering from consumption (tuberculosis) and possibly syphilis as well. His illnesses inhibited his contact with women and doomed him to an early death. While Keats himself was not yet ill when he wrote "When I have fears," he had just recently seen his brother succumb to tuberculosis, making him strongly aware of his own mortality. There is a persistently regretful and morbid tone in his verse, as in "When I have fears," where he "stands alone" and thinks, "until love and fame to nothingness do sink."