The phrase “O sylvan Wye” is a direct appositive to “thee.” The Wye is a river that runs near Tintern Abbey; “sylvan” means “wooded.” So it is the river Wye, an element of nature, that is the “wanderer in the woods.”

How oft in spirit have I turned to thee O sylvan Wye?

If this Be but a vain belief, yet, oh! how oft, 50 In darkness, and amid the many shapes Of joyless day-light; when the fretful stir Unprofitable, and the fever of the world, Have hung upon the beatings of my heart, How oft, in spirit, have I turned to thee O sylvan Wye!

What does Wordsworth mean by the fever of the world?

This disregard for modern life is also shown in Wordsworth’s choice of words when describing the “fever of the world” and the “din of towns and cities.” Both “fever” and “din” connote unwantedly spoiling something, which therefore implies that Tintern Abbey, specifically, is a healer and nurturer – “How oft… have I …