The couple - who announced last week they were ending their five-year relationship - reportedly split when the former Playboy model realised the spark had gone out of their romance.
A source close to Jenny - who has a seven-year-old son, Evan - told America's OK! magazine: "Jenny said it became very Jerry McGuire-esque, in the sense that she had this had this great guy, and he loved her kid, but they were no longer hitting their stride.
"She said when she and Jim worked, they worked - and when they stopped working, it was done, just like that.
"She just looked at him and was like, 'You love me like a family member that's never going anywhere, not like a woman who you can't wait to make love to.' "
According to the insider, Jim agreed he was in love with Jenny and Evan "as a package" but he "couldn't get his mind right, he wanted to, but somewhere along the way, he lost it."
According to the insider, the former couple have vowed to stay friends and the actor is still making daily visits to see Evan.