The Beatles icon admits he struggled to tell the New York socialite his real feelings after they started dating, so poured out his heart on Scared, a song which appears on his upcoming album New.
He tells Britain's The Sun, "It can be quite hard to say 'I love you' to someone. I felt this with Nancy but to say it wasn't that easy.
"There are a couple of tracks about Nancy and Scared is one of them."
MCCartney has previously penned tracks for the major loves of his life, including And I Love Her for actress Jane Asher, My Love for his beloved first wife Linda, and Heather, the track inspired by his ill-fated marriage to Heather Mills.
The musician and Shevell wed in 2011.