Into his mind the turbid ebb and flowOf human misery; we (17-18)
Here the speaker matches the all-important water imagery in this poem with the misery he sees in the world. Human suffering becomes its own kind of ocean, replacing the "Sea of Faith" that is gone forever. The fact that he puts these thoughts in the mind of Sophocles suggests that this is nothing new, that misery has always been a part of human life.
Hath really neither joy, nor love, nor light, (33)
Our loneliness in this dark and sad world is a big part of why humans are so miserable. Or at least that's what our speaker thinks. He emphasizes that things might look okay, but really all happiness is an illusion. The reality is just emptiness and pain.
Nor certitude, nor peace, nor help for pain; (34)
This is an especially brutal line, drying up any last shreds of hope we might have had about the world. As the speaker puts it, the problem isn't just that life is hard, it's that it's so uncertain. We can't count on anything. Except, of course, pain. We know it might seem like we're overdoing this point, but it's such a crucial part of what this poem is about—you can't get to the core of this poem without acknowledging how grim it is, don't you think?