Take this kiss upon the brow! And, in parting from you now, Thus much let me avow— (1-3)
The speaker voluntarily leaves the women and isolates himself. It's almost like he knows he has an important journey ahead and must go it alone. Then again, perhaps he wants to isolate himself from the woman who has told him life is a dream. We mean, that's not exactly a compliment.
I stand amid the roar Of a surf-tormented shore, (12-13)
The speaker is alone, on a very loud ("roar"), dangerous beach ("surf-tormented"). The noisy, scary setting reflects some of the turmoil and frustration of being alone, and the big scary thoughts that are roiling around inside his head.
How few! yet how they creep Through my fingers to the deep, While I weep—while I weep! (16-18)
Even the sands want no part of the speaker's plans; they desert him ("creep / through"), almost as if they, too, realize that his quest for understanding must be undertaken in solitude.
O God! can I not grasp Them with a tighter clasp? O God! can I not save One from the pitiless wave? (19-22)
There are several images of isolation in these lines. The letter "I" is repeated twice, and line 22 begins with the word "one." Overkill? Maybe. After all, we know he's alone, but apparently the point needs to be driven home.