Earlier this week, Ashton and the Ted star were spotted kissing at a party, with reports stating that Kunis has 'practically moved in' to Kutcher's Hollywood home.
Sources 'close' to the Ghost actress have since revealed that Moore - who ditched Kutcher after cheating reports - is struggling to see him move on from their marriage.
An insider told Radar Online: "Demi is devastated that Ashton has moved on so quickly and is dating Mila. She is beyond heartbroken.
"Even though Ashton cheated on her all the time, it was with random girls, it wasn't a relationship like it is with Mila and that is what is killing Demi now.
"Seeing photos of him kissing another woman is too much for her to handle, it's like having her nose rubbed in it time and time again."
The source also stated that Demi was hoping for a reconciliation with the Two And A Half Men actor before his blossoming relationship with Mila.
"She kind of knew it was definitely over between them, but I think she was still holding out a little hope for a reconciliation," they explained.
"Now he's actually dating someone else she knows that's not going to happen."
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