I measure every Grief I meet
This is one bummed out speaker, and yet, there's hope. Sure, she seems a bit preoccupied with her own grief and suffering—always comparing it to that of others and such. But she also is heartened by her belief that remembering the suffering of others and seeing how hers stacks up will bring her a "piercing Comfort" (35).
And we're betting she's also comforted by her belief in God, which comes into play at the end of the poem, when she mentions "Calvary." Calvary hill was the site of Jesus' crucifixion in the New Testament, and as she figuratively passes by it in her mind, it gives her a kind of solace. That's in keeping with the Christian belief that Jesus sacrificed himself to earn salvation for the world. Perhaps our speaker finds some relief from her grief in reminding herself of that every once in a while.
The speaker has a vivid imagination—much of her time in the poem is spent wondering and imagining what others are thinking and feeling. But while she's really into observing other people, she's not a know-it-all by any means. She admits that she may be wrong about what other people are experiencing, and that ultimately that's a-okay. In fact, she suggests that the feeling of not knowing—a.k.a. ambiguity—actually makes it possible for her to imagine that someone shares her feelings. And it's this imagining that makes her feel better.