The Glee star reveals her album was complete when label bosses asked her if there was anything she'd like to add.
She tells Elle magazine, "I was like, 'I might regret it if I don't'."
She chose to return to the studio and record the song You're Mine, adding, "It makes me so happy, this song. It makes me think so much of Cory. It was ours. When I think if him, I play this."
Monteith died from a drug overdose in July (13).