Two that spring to mind -"Let me down easy" by Ralph McTell, and "About the Children" by Tom Paxton.

When my first marriage broke up, I wrote this (don't know how to do music notation, so just the lyrics):

USED TO BE

Used to be I'd write you love-songs, just to see you smile,
Used to buy you foolish gifts, to make you laugh awhile,
Used to be we'd stop right there, and make love on the floor,
That's just the way it used to be, but never any more
-Any more.

Used to be I'd sing for you, and we would harmonise,
The songs were soft and sad, but there was laughter in our eyes.
But gin and late nights cracked that voice that rang like Lady Day,
And one too many hopeless vigils took my tunes away,
-My tunes away.

We killed all the love that we once lived for.
We chewed it up and spat it on the ground.
In the past, we bruised and bent and battered it,
But now we stepped right on in there and shattered it.
So now it's down to maintenance,
To weigh a marriage in pounds and pence,
And head for separate lifeboats now the ship is going down
-She's going down.

And Sunday summer evenings now aren't what they used to be,
I stay alone at home most times, and sit and watch TV,
And maybe I drink too much wine for our talks to go too deep,
And nights when you come home at all, we go to bed to sleep,
-Just to sleep.

Used to be my sad songs moved you, now you sit dry-eyed,
We've changed our ways: I think I liked you better when you cried.
Could be that we loved too early, maybe learned too late,
When love turns to indifference, it hurts much more than hate,
-So much more than hate.

It's no use blaming you, you're just the piper.
I called the tune and paid you in advance,
And now, with all these crowds of people watching,
-I'm scared to show that I don't know the dance.