[…] as if, facing them,the undertow of everything sufferedwelled up in the soul…I don't know! (2-4)
When we face "them" (the blows) every single thing that has ever happened to us, every single suffering we've ever been through, comes right back up to the surface. It's like when you bite your tongue and then, since it's swollen, you keep biting it more and more, and every time it hurts worse.
And man…. Poor…poor! […] (13)
The only adjective the speaker can think of to describe the man is "poor," and the word is repeated slowly, exclaiming, to show just how much man has suffered in his time on this planet. A lot of pain is packed into that one, repeated word.
[…] and everything livedwells up, like a pool of guilt, in his look. (15-16)
The thing about these lines is that suffering is transformed into guilt. Up to here all of the water-welling up has been suffering and experience. Now it's a pool of guilt, as if somehow the poor suffering man in the poem is responsible for his own depravity. That's a fine how-do-you-do. This might have something to do with religious ideas that our suffering is because of our guilt.