The couple, who had been dating since 16 years, decided to separate because Hamm, 44, declined to have a committed relationship with the "Kissing Jessica Stein" star, reported Aceshowbiz.
"The relationship just ran its course. Jennifer realized that she and Jon don't want the same things. So they agreed that it was time to take a step back," a source said.
Hamm and Westfeldt, 45, have not been photographed together since January, before he entered a 30-day rehabilitation program for alcohol abuse.
"I don't have a driving force to have a baby. That said, I'm in a committed relationship, and if it ever came up, I'm not ruling it out.
