As mentioned, there are a lot of country/western songs of the kind, although it would be hard to say whether on the basis of "broken hearts per song" the theme is better represented in CW than in folk. A difference might be that in folk(ish?) lore often someone gets killed (usually murdered?), while in C/W it's more likely just a "parting of ways."

While I may have missed some in the folk area, C/W seems to include many more that are "celebrations" rather than "lamentations."

"Thank God and Greyhound (She's Gone)" is a C/W of the latter kind that comes to mind and that's well known, but not well enough (by me) to quote lyrics or citation.

Mike Cross also does one called "Best Friend" that starts off:

The best friend I ever had has run off with my wife,
And left me here alone to face the problems of my life.
The best friend I ever had has stole my wife somehow,
I never liked him much before, but Lord I love him now.

...
He didn't have to sneak her off, I would have helped her pack,
But he'll still be my best friend, Long as he don't bring her back.

He's the one who'll get to feel her warm and tender touch,
But if she treats him like she did me, he won't feel it much.
In time he'll learn to understand, the nature of his luck,
And he'll love her like a 'possum on the higway loves a truck.

(verses may be out of order, and aren't complete, but ...)

Full lyrics for "Best Friend" may have been posted here previously, but I don't have an instant link handy.

Cross also has done one called just "Bill," IIRC, that's an exceedingly warped "funeral eulogy" that might qualify as the ending of a relationship, and may also have been posted in another thread.

John