If I had not met the red-haired boy whose father (1)
This line might not seem like it has much to do with identity, but it's the beginning of a very long sequence of events that helped shape who our speaker is. It all begins with a red-haired boy.
and if I had written anything else at the top of the examination form where it said college (10-11)
The choices the speaker made a long time ago, in his youth, affect who he is now, even if they didn't seem like a big deal at the time. So, straighten up and fly straight, all you young Shmoopers out there!
I would not have found myself on an iron cot (19)
This is the point in the poem where the speaker starts to think about himself (finally). He's been chatting about all these other people, and now he's taking the opportunity to consider exactly who all of this makes him.
with my head by the fireplace of a stone farmhouse that had stood empty since some time before I was born (20-21)
The speaker is considering how the farmhouse relates to his own personal history. Even this building is somehow related to his sense of himself. ("You're so vain, you probably think this farmhouse is about you…")
I would not have travelled so far to lie shivering (22)
The speaker has come from far away to be in this place, and it seems to have cost him his health. Why go through all that trouble? He might have come back to what he considers home to better understand himself.
thinking I knew where I was when I heard them fall (29)
Seems like Merwin could have substituted "where I was" for "who I was." Our speaker does all of this soul-searching during the poem, and by the end he seems to have a better, but still not perfect, sense of who he is. He thinks he knows, but that's a far cry from certainty. That checks out, though. Do we truly, perfectly, 100% know ourselves? Haven't you ever done anything to surprise yourself? Then you'll probably agree with the speaker here.