When I look behind, as I am compelled to look before I can gather strength to proceed on my journey,
Here, the speaker reflects on his past before he can move forward.
The idea of "look[ing] behind" in order to "gather strength" is one figurative way to show how we all mature and develop on a personal level through life experiences. The speaker acknowledges where he came from and everything that led him to where he is today.
He also recalls past events in order to grow from those experiences and apply that knowledge as he "proceed[s]" (or embarks) on his future travels. (We live and we learn, right? Well, ideally.)
I see the milestones dwindling toward the horizon
These lines provide on-the-ground details related to the last passage, as if the speaker is looking back over a timeline marked by substantial events that have previously unfolded.
The enjambment from line 11 (ending with "dwindling") to line 12 (starting with "toward") gives us a feeling of backward motion promptly followed by forward motion.
Crafty ol' Kunitz is using the form of the poem to enact a push-pull effect between the past and future.
Word choice is worth noting in line 11: "milestone" can be literally translated to mean a stone functioning as a milepost. In this way, "milestone" indicates great distances and periods of time passing.
"Milestone" can also symbolize significant stages or turning points in life. Kunitz plays with this dual meaning to further his argument: While many things fall away over time, a part of us is always aware (or conscious) for the duration of the ride. (See the "Symbols: Stones" section for more.)
and the slow fires trailing from the abandoned camp-sites,
Similar to the last section, we're given a bird's eye view of a landscape filled with dwindling fires to represent (metaphorically) all the events that have transpired during the speaker's long life.
Unlike "milestones," "slow fires trailing" is a more somber image (you know, flames, smoke, maybe some brimstone) that connotes loss and leaving old places ("camp-sites"), people, or things behind.
over which scavenger angels wheel on heavy wings.
Just when the poem was dishing out solid, concrete images, it shifts back into metaphysical territory. The speaker sees supernatural entities picking things up from his past.
A "scavenger" is an animal that searches through and collects discarded material. So, in this context, "scavenger angels" are acting as forces from another realm, collecting significant things from the speaker's past for him to take with him on his journey.
Although this is a weighty scene, Kunitz has constructed a dynamic and exciting analogy, showing how we must consciously pick and choose elements from our past in order to continue moving forward. (And scavenger angels are a pretty rad way to say that.)
The verb "wheel" in line 16 might seem like it came from left field. But it captures the cyclical nature of change that Kunitz is going for, with definitions like, "to turn, rotate, or revolve around an axis." (See the "Symbols: Turning" section for more on that.)