The speaker is dying, and the poem is unsure whether the cause is a disease or grief. Everything seems to have started when someone they love left, and the speaker admits they can no longer tell whether the tightness in the chest is physical illness or heartbreak. That blurring is the heart of the poem.
The tone is calm and resigned rather than panicked. The lines are short and plain, and "Time is trivial now" suggests the speaker has stopped fighting. Thoughts of death come in pairs, "my death; / thoughts of yours," which hints that the loved one may already be dead, though the poem never says so outright. The speaker expects to be welcomed and ends with an invitation: "meet me by the gates." The mood is quiet, sorrowful, and strangely tender, because death feels less like an ending than a way of getting back to someone.