Idont know what this stanza means and need with the analysis!

my heart aches, and a drowsy numbness pains
my sense, as though of hemlock i had drunk,
or emptied some dull opiate to the drains
one minute past, and lethe-wards had sunk:
'tis not through envy of thy happy lot,
but being too happy in thine happiness,—
that thou, light-winged dryad of the trees
in some melodious plot
of beechen green, and shadows numberless,
singest of summer in full-throated ease.