Just watched Glee Season 2 Episode 20, and Rachel sang this song during the Prom and I love it so much. So I googled it up and found the song. It's call Jar of Hearts by Christina Perri.
She kinda reminds me of Avril Lavigne. Anyway, here's the song. Hope you all like it too. Love the dance a lot. Really brings out the emotion, the feeling that are expressed through the song.