In the first stanza, the speaker says that he used to see elements of nature as though they were "Apparelled in celestial light, / The glory and the freshness of a dream" (lines 4-5). Everything in nature seemed to him to be almost holy or divine, bathed in a beautiful light that made it all seem glorious and fresh, as though he were actually living within a vivid dream. However, he no longer feels this way. He says,
It is not now as it hath been of yore;—
Turn wheresoe'er I may,
By night or day.
The things which I have seen I now can see no more. (6-9)
In other words, he no longer sees nature in the same way that he used to; things are no longer bathed in the beautiful light that they once were. It seems that he has changed somehow, that he has grown up and lost his innocence, and this has forever changed the way he sees the world around him, including the natural world. Things are still beautiful: the rose is still "lovely" and the stars' reflection in the water is "beautiful and fair," he says,
But yet I know, where'er I go,
That there hath past away a glory from the earth. (17-18)
Though he can still recognize the beauty of nature, that special sense of its being vivid and fresh and glorious as a dream or of being bathed in a "celestial light" is gone. The conflict exists between how he used to see nature when he was younger and how he sees it now: it has changed, and he seems saddened by his inability to view nature with his more youthful, innocent eyes.