And the heart must pause to breathe, (7)
It seems like he's talking about taking a quick breather (like you do when you exercise), but there's something more sinister going on here. The idea of pausing, in a poem that contains several references to death, makes us think of the permanent pause of death.
And love itself have rest (8)
Again, an apparently "normal" line has a darker meaning. The word "rest" in particular is troubling. Just think of those common epitaph words—"Rest in Peace"—to catch our drift. Love must rest, sure, but it sounds like a permanent rest.
And the day returns too soon, (10)
The return of day is a sign of hope and life. Perhaps it's not really the end after all. Or maybe the speaker is thinking in some bizarre way about his soul's rebirth in another world.