The speaker begins by being almost painfully gracious. They say they will never blame the person who left, and they grant that person every right to love or to pull away. The old-fashioned "ne'er" and "thee" give this opening a formal, dignified tone, as if the speaker is trying to hold themselves together and behave well.
Then the poem turns on "But I wish." The ending is small and aching: when you left, you could have left a little of me. The speaker seems to feel emptied, as though the other person took not just themselves but a part of the speaker too. Perhaps they want a trace of closeness, or a piece of their own self back. The poem doesn't say which.
The broken, dash-heavy lines feel like hesitant speech, and the mood is quiet, wistful sadness without anger. The speaker accepts the loss, yet cannot quite hide the hurt.