I've been listening to this amazing singer named A Fine Frenzy, and some of her songs are IMHO reflect some of the emotions & struggles that Zero & Yuuki are going through. For instance:
This song pretty much talks about Yuuki's feelings for Zero, especially with the lines "Goodbye, my almost lover. Goodbye, my hopeless dream. I'm trying not to think about you. Can't you just let me be..."
If Chapter 51 can have a theme song, then this would definitely be it. Yuuki knows in her heart that she's better off being with Kaname, yet it's still hard for her to completely forget her love for Zero.
This is Zero's song, his inner turmoil over his doomed-from-the-very-start relationship w/ Yuuki. The first lines "Dunno what to do anymore. I've lost the only love worth fighting for. And I'll drown in my tear storming seas. That would show you, that would make you hurt like me." is exactly what Zero's going through right now.