While I weep—while I weep! (18)
The speaker's weeping tells us a lot. It's clear that his attempts to prove that reality is real have ended in defeat. And he's really unhappy about it, which is why he tells us twice that he is weeping. Just so we don't forget. Not like you can't hear him all the way to China, anyways. Dude's blubbering.
O God! can I not graspThem with a tighter clasp?O God! can I not saveOne from the pitiless wave? (19-22)
While these lines are questions, they are questions that imply negative answers: the speaker cannot hold the sand with a "tighter clasp" or save any grains from the "pitiless wave." He is failing, as we've been led to believe throughout the poem, and here it seems like he's admitting defeat before the battle's even over with.