The Song I Didn't Write

I can’t wrap my head around
The notion that the present is a gift.
You know me well enough by now
To know I know that your devotion
Has a tendency to shift. 

I can wait for you forever
If that’s what you want. 
I can be a fountain for you
Always filling, always spilling. 

I burnish every surface in the house
But still I’m haunted by the dust. 
How much of it is your skin? 
How could I begin again? 

You know the words to all my songs
And you prove it to me every night. 
But could you guess the lyrics
To the song I didn’t write?