This poem uses copious parallelism to keep its stanzas (and its speaker) circling around inescapable regrets and pains.
In the poem's first three lines, for example, parallelism immediately draws attention to the speaker's past and current relationship with "desire":
I have desired, and I have been desired;
But now the days are over of desire,
The anaphora on "I have desired" and "I have been desired" stresses that the speaker has experienced both sides of desire: wanting and being wanted. Epistrophe (the repetition of "desired"/"desire" at the ends of lines 1 and 2—also an example of polyptoton) emphasizes the fact that, while desire was once the speaker's whole life, that's all over now.
Lines 6-7 also use parallelism to emphasize how far the speaker feels from her former passions:
Longing and love, pangs of a perished pleasure,
Longing and love, a disenkindled fire,
And memory a bottomless gulf of mire,
And love a fount of tears outrunning measure;
The powerful anaphora here first emphasizes the fact that the speaker's "longing and love" are nothing but a "disenkindled fire" now, a burned-out shell of what they once were. Then, those repeated "and"s pile disappointment on disappointment.
A similar pile of parallel disappointments appears in lines 12-13:
Drop by drop slowly, drop by drop of fire,
The dross of life, of love, of spent desire;
The repetition of "drop by drop" evokes the slow, painful draining of passion and life from the speaker's heart. And the anaphora on "of" makes it clear that the word "dross" (which means useless trash) equally describes the speaker's "life," "love," and "desire." In other words, looking back on her life and all the time she's devoted to her love affair, she feels both drained and cheated.
The poem's parallelism rises to a high dramatic pitch in the closing stanza:
Oh vanity of vanities, desire;
Stunting my hope which might have strained up higher,
Turning my garden plot to barren mire;
Oh death-struck love, oh disenkindled fire,
Oh vanity of vanities, desire!
The anaphora on the speaker's repeated cry—"oh"—imbues these lines with emotion and drama, revealing how utterly forlorn the speaker feels thinking back on her wasted time and energy. And the parallelism on the verbs "stunting" and "turning," for one last time, emphasizes that love has changed the speaker, and only in the worst ways.