AceShowbiz -

The real reason that prompted Jon Hamm and Jennifer Westfeldt's recently announced separation has been revealed. A source claimed the former couple, who had been together for 18, years actually had been broken up for a few months now. The source, moreover, informed ET the 45-year-old writer/director got tired of the 44-year-old "Mad Men" star's "party-heavy lifestyle."

"Jon and Jennifer really fought hard to make their relationship work, Jennifer more so than Jon," said the source. "They were together for many years, but in the end, it was too much work. It's no secret that Jon likes to go out and leads a very party-heavy lifestyle. And Jennifer was just tired of that." The source then added, "She wanted him to grow up ... and he never wanted to. She was tired of being his mother."

Hamm and Westfeldt began dating in 1997 and starred together in the 2011 film "Friends with Kids". In a joint statement announcing their separation, the couple was quoted as saying, "With great sadness, we have decided to separate, after 18 years of love and shared history. We will continue to be supportive of each other in every way possible moving forward."