SUNNY HILL.

BTOB- father
Westlife- you raise me up
Ukiss- someday
Ukiss- believe
Taylor swift- never grow up

I don't think i know anymore.. yeah, it's hard to find sad songs that aren't related to love..

I never thought of Goodbye as angst. I always thought it was inspiring haha

Have you tried:
Ed Sheeran: Drunk; Little Bird (legit about a bird); The A Team (prob the best one, tbh)
Kodaline: High Hopes; All I Want
Emile Sande: Clown
U-Kiss: Someday

There are the ones I know that aren't based on love. I know a couple with an angsty feel (idk if they are about love / some mention it briefly), do you want those?

Have you tried Rain Sound, Coma, Teardrops-CNBlue or The Otherside? The Otherside is by a singer named Alana Grace she's really good ^^

Oh and there's another I like called Kagi ga aknai by Suitei Shoujo.

Father - BTOB!