The former couple announced their break up on Monday, telling ET in a joint statement, "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."
However, a source tells ET that the two have actually been broken up for a few months now. According to the source, the 44-year-old Mad Men star's "party-heavy lifestyle" got to be too much for the 45-year-old writer/director.
"Jon and Jennifer really fought hard to make their relationship work, Jennifer more so than Jon," the source says. "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."
"She wanted him to grow up … and he never wanted to," the source adds. "She was tired of being his mother."
In March, Jon completed treatment in a rehab facility
, where he sought help in battling an alcohol addiction. During the obviously tense time, Jennifer stood by side. "With the support of his longtime partner Jennifer Westfeldt, Jon Hamm recently completed treatment for his struggle with alcohol addiction," his rep told ET about his 30-day stay.
While Jennifer not surprisingly stayed mum on Jon's rehab, in April, the two did shoot down a tabloid report claiming she dumped him before he entered treatment. "The story that appeared in this week's edition of In Touch
magazine is not true," the couple's rep said in a statement to People
. "The only gentleman Jennifer visited regularly in Connecticut was Jon, while he was in rehab. They continue to ask for the public's understanding and sensitivity during this challenging time."
In 2011, the handsome actor talked to ET about how he and Jennifer were able to keep their relationship going after all these years. Jon and Jennifer first started dating in 1997, after meeting each other at a party.
"We go to great lengths to keep our lives the same that they have always been ... part of it being a little bit boring," he admitted. "That helps if you're not stealing jewelry or knocking over liquor stores, or getting in fights -- then nobody wants to take a picture of us."
"We're like an old married couple," Jennifer added.