Parallelism is one of several forms of repetition in this poem, all of which add to its sense of passion and forcefulness.
In the first stanza, for example, the speaker repeats the phrase "For those of us who" (which is also an example of anaphora), followed by a phrase describing something that defines the life of "those" whom the speaker is addressing. There's also parallelism in the phrases "coming and going," "inward and outward," "before and after." Together, all this parallelism builds up the first stanza's intensity, creating a steady, progressive rhythm that pulls readers forward through the poem.
But the strongest parallelism comes in the poem's third stanza, which features parallel phrase after parallel phrase. Every two lines mirror the two preceding lines. Take lines 25-28 for example:
And when the sun rises we are afraid
it might not remain
when the sun sets we are afraid
it might not rise in the morning
The repetition of sentence structures draws attention to the fact that regardless of what the sun does, these people feel afraid. Their fear isn't really dependent on the sun, even though that's what it seems like at first; their fear is constant, and nothing the sun does can assuage it.
The speaker repeats this format throughout the stanza, presenting two opposing states and insisting that people's fear exists in both. This device is also an example of antithesis, which hammers home the point that there's no safe place for the people the speaker addresses—that there's no escape from their fear.
In lines 37-41, the parallel grammatical structure is varied just slightly, but only enough to add further emphasis:
and when we speak we are afraid
our words will not be heard
but when we are silent
we are still afraid
The addition of line 39 ("nor welcomed") breaks up the repetition a bit, just enough to draw out the feeling of what the speaker is saying: that even if these people's words were heard, they wouldn't be welcome. They are being "unheard" on purpose. By breaking up the final phrase ("but when we are silent / we are still afraid") into two lines, the weight of stanza three falls firmly upon that final admission: that no matter what happens, these people will be filled with fear.