…Oh, at last, when you return through the three Pa districts, Send me a message home ahead!
The speaker addresses her husband directly in these lines, as she does in the rest of the poem, asking him to send her a message telling her that he's arriving. Her direct address to him in the form of a demand ("Send me a message") underscores her desperation to see him.
Here the speakers projects into the future, imagining her husband's return through "the three Pa districts." The Pa districts are in the area of the ancient city of Changgan.
These lines reflect the speaker's hope. Even though it's been months and months since he's left, the speaker still believes that he'll come back one day. For her sake, we hope so.
And I will come and meet you and will never mind the distance, All the way to Chang-feng Sha.
The speaker tells her husband that she'll come and meet him at "Chang-feng Sha" when he sends her a message of his arrival. Chang-feng Sha is an area that's a few hundred miles away from Changgan. So the speaker herself is willing to journey far in order to welcome her hubby home. What a lovely (and lonely) wife she is.
Even though these final lines evoke the speaker's hope for seeing her husband again, there's something sad about it.
That's because her husband hasn't actually come back yet. These final lines reflect the speaker's wishful thinking.
So, while these lines reflect the speaker's hope for seeing her sweetheart again, they call attention to the fact that he hasn't actually returned.
The final lines of the poem, in other words, leave us hangin'. Will this speaker see her husband again or not? We just don't know. This poem is one big cliffhanger.