What does "Lethe-wards had sunk" mean?

Context:

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.

O, for a draught of vintage! that hath been

Cooled a long age in the deep-delved earth,

Tasting of Flora and the country green,

Dance, and Provencal song, and sunburnt mirth!

O for a beaker full of the warm South,

Full of the true, the blushful Hippocrene,

With beaded bubbles winking at the brim,

And purple-stained mouth,

That I may drink, and leave the world unseen,

And with thee fade away into the forest dim.

Fade far away, dissolve, and quite forget

What thou amongst the leaves hast never known,

The weariness, the fever, and the fret

Here, where men sit and hear each other groan;

Where palsy shakes a few, sad, last grey hairs.

Where youth grows pale, and spectre-thin, and dies;

Where nut to think is to be full of sorrow

And leaden-eyed despairs;

Where Beauty cannot keep her lustrous eyes,

Or new Love pine at them beyond to-morrow.

Away! away! for I will fly to thee,

Not charioted by Bacchus and his pards,

But on the viewless wings of Poesy,

Though the dull brain perplexes and retards.

Already with thee! tender is the night,

And haply the Queen-Moon is on her throne,

Clustered around by all her starry Fays;

But here there is no light,

Save what from heaven is with the breezes blown

Through verdurous glooms and winding mossy ways.

I cannot see what flowers are at my feet,

Nor what soft incense hangs upon the boughs,

But, in embalmed darkness, guess each sweet

Wherewith the seasonable month endows

The grass, the thicket, and the fruit-tree wild --

White hawthorn, and the pastoral eglantine;

Fast fading violets covered up in leaves;

And mid-May's eldest child,

The coming musk-rose, full of dewy wine,

The murmurous haunt of flies on summer eves.

Darkling I listen; and for many a time

I have been half in love with easeful Death,

Called him soft names in many a mused rhyme,

To take into the air my quiet breath;

Now more than ever seems it rich to die,

To cease upon the midnight with no pain,

While thou art pouring forth thy soul abroad

In such an ecstasy!

Still wouldst thou sing, and I have ears in vain --

To thy high requiem become a sod.

Thou wast not born for death, immortal Bird!

No hungry generations tread thee down;

The voice I hear this passing night eas heard

In ancient days by emperor and clown:

Perhaps the self-same song that found a path

Through the sad heart of Ruth, when, sick for home,

She stood in tears amid the alien corn;

The same that oft-times hath

Charmed magic casements, opening on the foam

Of perilous seas, in faery lands forlorn.

Forlorn! the very word is like a bell

To toll me back from thee to my sole self!

Adieu! the fancy cannot cheat so well

As she is famed to do, deceiving elf.

Adieu! adieu! thy plaintive anthem fades

Past the near meadows, over the still stream,

Up the hill-side; and now 'tis buried deep

In the next valley-glades:

Was is a vision, or a waking dream?

Fled is that music -- Do I wake or sleep?