So, we'll go no more a roving So late into the night, Though the heart be still as loving, And the moon be still as bright. (1-4)
We can sense some dissatisfaction here. Although his heart is working just fine, and even though the moon is still just as inviting, there's something that's no longer satisfying to him. He's very vague about what that is exactly, however. Sigh.
For the sword outwears its sheath, (5)
Let's pretend the speaker is comparing himself to the sword. He's no longer satisfied with the metaphorical "sheath" that is his life. He needs to find a different one that will fill that gaping void he feels.
And the soul wears out the breast, (6)
The speaker gives his dissatisfaction a more spiritual cast. He compares himself to a soul that needs a new body. This makes us think of him as an immaterial, floating being that has no proper home. Yikes. Who knew dissatisfaction could be so bad?
And the heart must pause to breathe, (7)
Pausing to take a breath? We're guessing this isn't because the speaker has just finished working out. He's pausing to breathe, but we think he means pausing and sighing and reevaluating his life. That's what dissatisfaction does.
And love itself have rest. (8)
This is just like line 7. It's not that the speaker is so tired from all his loving. No, no, no. It's "rest" in the sense of a break, a cessation for good, because love is no longer a source of satisfaction. That's sad.