"Sonnet 78" is part of Spenser's Amoretti, a sonnet sequence of 88 sonnets, which, along with Epithalamion (1595), is believed to memorialize his courtship and marriage to Elizabeth Boyle. "Sonnet 78" is a typical Spenserian sonnet, which blends the Shakespearean and Petrarchan (Italian) forms, using three quatrains and a couplet, but employing a linking rhyme scheme--abab/bcbc/cdcd/ee.
"Sonnet 78" centers on the poet's lost (presumably, temporary) lady, and consistent with Spenser's love of nature, uses metaphor draws from nature to carry his despair:
Lackyng my love I go from place to place,/Lyke a young fawn that hath lost the hynd;/And seeke each where, where last I saw her face./Whose ymage I carry fresh in mynd (ll. 1-4)
These lines appropriately establish the sonnet's dominant theme, the poet's loss of his lover and his own state of being lost. He compares himself, for example, to a fawn that has lost its mother (the hynde)--thereby losing not just a companion but his sustenance. The lines end with an equally important theme: the poet's ability to hold the image of his beloved in his mind.
Carrying forward the metaphor of the poet as the lost fawn seeking its mother, the poet seeks her "in the fields" and in her "bowre" (her place of safety), and although he is unable to find her, the field and bower are "full of her aspect," that is, the poet sees her image everywhere--because her image is still "fresh in mynd," he "sees" evidence of her wherever he looks, but that is not good enough.
In the third quatrain, the poet realizes that his eyes are playing tricks on him:
and when I hope to see theyr trew object [that is, the missing lover],/I find my selfe but fed with fancies vayne. (ll.1-11)
In essence, the poet's eyes are creating images that turn out to be completely false, and he realizes that he is not going to find his missing lover with his eyes. The poet's eyes, rather than focusing on what is lost, are focusing on what the poet wants to find, creating "fancies vayne" that simply lead him away from the true goal.
The poet's inability to "find" his lost lover by looking for her is resolved in the sonnet's couplet:
Cease then myne eyes, to seeke her selfe to see,/And let my thoughts behold her selfe in mee. (ll. 14-16)
The poet finally understands that his missing lover, if not to be found in the physical world, can be found in his heart, in his memory.