Spenser's Amoretti Sonnet 75 is structured with three quatrains (four line stanzas) and a couplet to comprise the fourteen lines of the sonnet form--as prescribed by the original sonneteer, Petrarch--in the Spenserian rhyme scheme ababbcbccdcdee (abab bcbc cdcd ee), with linking of rhyme/thought at concatenated lines bb cc (4-5, 8-9). Starting in the middle of the sonnet, the third quatrain introduces the sonnet paradox: "you shall live by fame; / My verse, [you] ... shall eternize." A paradox is an idea or concept that appears to be false ("baser things devise / To die ... but you shall live"), but is actually a truth of some sort: "My verse, [you] ... shall eternize."
The concluding couplet (two rhyming lines) resolves the conflict between mortal death and immortal life by explaining "Where whenas" all in the world shall in due time die, their love (the sonneteer and the lady with him, Elizabeth Boyle) "shall live" and be read by all through all time in his Sonnets, which will give renewal to all those who read them.
Quatrains one and two are a little trickier to analyze for structure. Spenser innovated a form of sonnet that allows for a logical progression of one subject without the introduction of a paradox. This is different from the Petrarchan form because it and the Shakespearean form have three subjects in all, one in each quatrain. This Petrarchan/Shakespearean form allows for two points of view and a paradox.
In contrast, Spenser's innovation allows for one subject carried through all three quatrains for a logical progression with one point of view and without paradox. The analytical question is: Does 75 represent the logical progression of one subject in the form of a conversation or does it represent two points of view and a paradox before the resolving couplet?
Having defined the question in this way, it seems more clear that Sonnet 75 introduces two points of view and a paradox before the couplet. This means that, structurally, there are two voltas (turns in the subject), one where the points of view change (line 5) ("Vain man, said she, that doest in vain assay,") and one where the paradox is introduced (line 9):
Not so, (quod I) let baser things devise
To die in dust, but you shall live by fame:
This analysis makes this sonnet structure the same as Shakespeare's Sonnet 18: fourteen lines; three quatrains; two voltas, lines 5 and 9; one paradox, quatrain 3; and one paradoxical resolving couplet, except for Shakespeare's unconcatenated (un-linked) rhyme scheme of ababcdcdefefgg (abab cdcd efef gg):
So long as men can breathe or eyes can see,
So long lives this and this gives life to thee.