Two songs come to mind immediately.

"There Are No Words" written and sung by American folk singer,
Kitty Donohoe. Written after the attacks on 9/11.
She actually sang it at one of the memorials. Can be heard on Youtube.

"If I Had Only Known" sung by Reba McEntire. Very sad song as well.

"The Promise" by Donovan - a very sad song of lost love.
I don't think he recorded this song on any album - written in more recent years. I heard him sing it in a concert in Monterey, California about 3 or 4 years ago. The lyrics are so beautiful but very sad.