Forgetfulness by Billy Collins.

Part 2 of 2.

Whatever it

is you are struggling to remember,
it is not poised on the tip of your tongue,
not even lurking in some obscure corner of your spleen.

It has floated away down a dark mythological river
whose name begins with an L as far as you can recall,
well on your own way to oblivion where you will join those
who have even forgotten how to swim and how to ride a bicycle.

No wonder you rise in the middle of the night
to look up the date of a famous battle in a book on war.
No wonder the moon in the window seems to have drifted
out of a love poem that you used to know by heart.