I have walked through many lives, some of them my own, and I am not who I was,
Straight out the gate, our speaker gets metaphysical. He has experienced "many lives" and changed as a result of those adventures.
The end of line 1, "many lives," and the entirety of line 2, "some of them my own," could be a literal references to reincarnation (living multiple lifetimes). Or, the speaker could be pointing out how expansive and colorful his past has been, in a metaphorical sense.
Kunitz leaves it open for interpretation, but one thing's for sure: this speaker is old and deep! And his hyper-awareness plays an important role throughout the poem. (Special note: There are no specific details assigned to the speaker, making him a universally relatable character, or an "everyman." Even the speaker's gender is not disclosed, but we'll stick to "he" for the sake of convenience here.)
By line 3, we know a clear change has taken place within the speaker, "I am not who I was." Not only has he left an old life (or lives) behind, but he's also observed how fleeting this crazy thing called "life" can be.
though some principle of being abides, from which I struggle not to stray.
In contrast to the last section, these lines focus on a permanent aspect of identity that does not change over time. The speaker uses a constant, or "principle of being," as a guide to follow on the road ahead.
Line 4 into line 5 contains a pretty abstract (but pretty important) passage packed with meaning: "principle of being / abides."
Let's take a closer look: the noun "principle" has definitions ranging from "guiding sense" to "resounding truth." The prepositional phrase, "of being," essentially stands for "of existence." In this context, "principle of being" is then a core sensibility, or a guiding light, that directs the speaker and shows him where to go.
The verb "abides" (endures or continues) is yet another clue that this guiding light is steadfast and unwavering.
This concept could be compared to a "soul" or "intuition." So, why doesn't Kunitz say that? Well, this poem is all about a universal understanding of the human condition. By implementing general terminology, Kunitz is making the poem relatable to the audience (or appealing to the masses), regardless of our backgrounds or belief systems.
In line 5 and 6, the speaker "struggle[s] not to stray," or tries to adhere to this core sensibility even under difficult circumstances.
One last thing: notice anything about how this poem sounds? It's pretty conversational, not anything super-formal or up-tight.
Check out "Form and Meter" for the scoop on how this is all put together.