Both answers are excellent. I have one small point to add: The effect of misusing our powers, or misdirecting our spiritual and emotional energy, is that we have created a lonely world.
The poet bemoans the spiritual disconnect we have wrought by living worldly lives. As sullymonster has said, we are too focused on materialism to be spiritual. Herin lies a paradox: we are both too immersed in the world (meaning worldy concerns) and too separated from it (meaning we've lost our spiritual connection with nature.
When the narrative persona says he's "rather be a pagan", he's not necessarily saying he wishes to revert to that time or that religion (this is evident in the fact that he calls it a "creed outworn"). What he does want, though, is to recover some of the wonder and awe that the pagans experienced in nature.
He says that were he a pagan, he would see glimpses of the gods (Proteus and Triton) that would make him feel less forlorn. To be forlorn is to feel sad and lost, to feel abandoned and alone. He uses the allusion to the gods to show us we are separated from God (in a poetic, not Christian sense), that everything is wrong (spiritually) with the way we live our lives.
What he's talking about here is the extreme spiritual and emotional isolation he (and we) have created by living worldly lives that are separated from nature. The cost to the individual is to feel forlorn: abandoned, separated, and utterly alone.