The star, who turns 50 next month, is said to have been 'devastated' over the shots of 29-year-old Kunis in her underwear.
Kunis recently being voted 'Sexiest Woman Alive' by Esquire is thought to have been "too much" for Moore.
A source told Heat: "Last week, when Esquire landed on her desk at home, Demi broke down.
"It's all too much for her. Ashton's replaced her. Demi has tried her best to move on, but now she feels she's back at square one again.
"Her confidence was already shattered, and seeing Mila like that has really affected her. Not many women have to deal with their ex's new girlfriend being branded 'The World's Sexiest Woman'."