Often these metaphors are complex. They have several levels of meaning, and they control the poem. This type of metaphor is called a conceit . Once you identify the conceit, the rest of the poem becomes easier to understand. Finally, don't expect these poems to be real love poems.
Nobody falls in love in three quatrains and a couplet.
Real love cannot be expressed in 140 syllables and through poetic conventions. These poems are about poetry. The love described should be understood as the occasion for, not the subject of, the poems. The first poem in Astrophil and Stella appears to be a call for originality. Be careful, though, even this is part of the convention started by Petrarch.