Let's see how it's turned out.
1. "Streetlights" by Ludo. A concert version, so the sound isn't great and the crowd at the beginning is very chipper. But the song sounds a bit melancholy in a rock ballad sort of way. Sure? 3/5.
2. "Into the Ocean" by Blue October. The lyrics are sadder than the music (I like "I'm as cold as cold as cold can be"), but it works. I like it. 4/5.
3. "Lose Control" by Evanescence. I very much liked the sad "oh oh oh oh" at the beginning of the song, but then it got to the chorus and just kind of got whiny. 3/5.
4. "Delicate" by Damien Rice. Very very melancholy. I looked up the lyrics and I LIKE. Totally works, especially around the 4-minute mark, where the vocals begin to gain more of a wailing quality. 5/5.
5. "This Lush Garden Within" by Black Tape for a Blue Girl. Well, this is a very long song, nearly 10 minutes. 2 minutes in, I'd call it more ominous than sad. And then the vocals come in, and I can't decide whether I love it or hate it. As it builds, it definitely is sadder, but in a very... crushing sort of way. It's fading away at 4:30. Is the song over? ...Apparently so, it's silence for the last five and a half minutes. I think I am going to say that it fits almost absolutely with my playlist, especially toward the end, so 4/5.
6. "The Islander" by Nightwish. I'm kind of digging the song. It's not as outright sad as the others (has an oddly triumphant tone to it) unless you think of it as the tragic sea shanty it is, then it kind of works. 3/5.
7. "Perfect World" by Simple Plan. I can't take this song seriously. It just makes me think of whiny teenagers. It actually contains the line, "Why is everything so hard?" 2/5 for effort, but I like my pop punk sardonic, not emo.
8. "Speaking a Dead Language" by Joy Williams. I saw this same poster use this song for a lot of different song recommendation threads, and every time expressed their love for it. I think it seeped into me, because although this is only the first time I've seen it, and there's nothing all that special about it, it's really working for me. Let's give it a 4/5.
9. "Macho, toki no nagare, hito" from Clannad. Wow. This totally works. It's kind of breaking me. 5/5.
10. "Chapter Four" by Avenged Sevenfold. OK, so I don't listen to this genre on purpose. I can't tell if this sounds sadder than other songs in the same genre. Someone who doesn't mind screaming vocals, tell me. I don't know how to rank it.
Well, certainly with this playlist, YMMV. (Warning: that link leads to TV Tropes and you may get stuck there forever.) And I know there are some songs by suggested artists that don't fit, even though the rest of the artist's work may. So I'm more hesitant about recommending this one than the post-apocalyptic one.
But, hey, there are 478 songs on here, and overall it looks like most of them at least kind of fit. And I'm pretty sure there are at least three of these I am going to go find for my own music collection shortly. Discovering new music ftw.