It's "Cinderella wa Damasarenai" from the Seishun Girls album (it's the last on that album). It's also one of my favorite AKB48 songs, along with Aitakatta, Sakura no Hanabira-tachi, Kimi wa Pegasus, MARIA, Seishun Girls, Nageki no Figure, and Korogaru Ishi ni Nare.