For this, for everything, we are out of tune; (8)
Humanity is "out of tune" with nature partly as a result of consumerism; the speaker characterizes modernity as a period of dissonance, a period in which humanity and nature make a discordant sound rather than a harmonious one. If modernity is out of tune, then the lines imply that being "in tune" is a characteristic of pre-modern society.
Great God! I'd rather be A Pagan suckled in a creed outworn (9-10)
From the speaker's vantage point, paganism is a thing of the past; the speaker says he would rather be a pagan, which implies that the pre-modern world is better than the modern world, that the society of the ancient pagan is less depressing than that of the "Great God" of Christian modernity.
So might I, standing on this pleasant lea, Have glimpses that would make me less forlorn; (11-12)
The poet tells us he's "forlorn" (i.e., sad, depressed, etc.). He implies that the present state of things – modernity – is inherently depressing; if he were a "Pagan suckled in a creed outworn" (9), he would feel "less forlorn." If he would only be "less forlorn," rather than "totally ecstatic," perhaps the past isn't that much better than the present. Maybe it's all in his head.