Place this clip right after the late confession is revealed, as the closing beat of the scene, then follow it with Sad Violin. For a different order, put Oh My God Wow just before it, so the first reaction comes before this line.
About this sound
This is a very short spoken line, lasting less than a second, in which a voice says it wishes it had known sooner. It feels wistful and slightly awkward, like a late admission that arrives after the moment has already passed, and it works because the regret is plain and unguarded.