First of all, this isn't an easy song to sing. The verses are written in the key of G and the song abruptly changes to the key of Bb for the chorus/hook. This song was just out of her range. She was already singing at the peak of her range in the beginning at the first key change (for the chorus). The second key change from Bb to B (half step) was the nail in the coffin for her, that's why it seemed so sudden that her voice gave out. And what made it worst, she was trying to sing it in her chest voice and when that wasn't successful, she had to try to make the transition back to her head voice to finish the song. It was a hard transition for her and it probably didn't feel comfortable either. Her head voice singing is not as supported as her chest voice singing therefore it's was not as full and it was getting sloppy because she lost control. Basically, she should have lowered the key. I'm sure her nerves played at part in it too.
