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.
Jon Hamm and Jennifer Westfeldt
Why Jon Hamm and Jennifer Westfeldt split after 18 years: 'She was tired of being his mother'
BEVERLY HILLS, CA - DECEMBER 18: Actors Jon Hamm (L) and Jennifer Westfeldt attend The PEOPLE Magazine Awards at The Beverly Hilton Hotel on December 18, 2014 in Beverly Hills, California. (Photo by C Flanigan/Getty Images)
NEW YORK, NY - JANUARY 05: Jon Hamm and Jennifer Westfeldt attend the 'Girls' season four series premiere after party at The Museum of Natural History on January 5, 2015 in New York City. (Photo by Jamie McCarthy/Getty Images)
NEW YORK, NY - DECEMBER 08: Jon Hamm and Jennifer Westfeldt attend 'Into The Woods' World Premiere - Outside Arrivals at Ziegfeld Theater on December 8, 2014 in New York City. (Photo by Rob Kim/Getty Images)
NEW YORK NY - OCTOBER 26: Jon Hamm and Jennifer Westfeldt sighting on October 26, 2014 in New York City.
(Photo by Josiah Kamau/BuzzFoto/FilmMagic)
LOS ANGELES, CA - AUGUST 25: Jon Hamm and Jennifer Westfeldt arrives at the 66th Annual Primetime Emmy Awards at Nokia Theatre L.A. Live on August 25, 2014 in Los Angeles, California. (Photo by Steve Granitz/WireImage)
NEW YORK, NY - APRIL 15: Jon Hamm and Jennifer Westfeldt attend the Opening Night After Party for 'The Library' at The Public Theater on April 15, 2014 in New York City. (Photo by Walter McBride/WireImage)
LOS ANGELES, CA - JANUARY 23: Jon Hamm and Jennifer Westfeldt are seen as they take their dog for a walk on January 23, 2014 in Los Angeles, California. (Photo by Bauer-Griffin/GC Images)
LOS ANGELES, CA - MARCH 20: Jennifer Westfeldt and Jon Hamm arrive at AMC's 'Mad Men' season 6 premiere held at DGA Theater on March 20, 2013 in Los Angeles, California. (Photo by Michael Tran/FilmMagic)
WEST HOLLYWOOD, CA - FEBRUARY 24: (EXCLUSIVE ACCESS SPECIAL RATES APPLY; NO NORTH AMERICAN ON-AIR BROADCAST UNTIL FEBRUARY 28, 2013) Actors Jon Hamm (L) and Jennifer Westfeldt attend the 2013 Vanity Fair Oscar Party hosted by Graydon Carter at Sunset Tower on February 24, 2013 in West Hollywood, California. (Photo by Larry Busacca/VF13/WireImage)
NEW YORK, NY - JUNE 20: Actors Jon Hamm and Jennifer Westfeldt attend The Cinema Society with The Hollywood Reporter & Piaget and Disaronno screening of 'To Rome With Love' at the Paris Theatre on June 20, 2012 in New York City. (Photo by Jim Spellman/WireImage)
68th ANNUAL GOLDEN GLOBE AWARDS -- Pictured: (l-r) Jon Hamm and Jennifer Westfeldt during the 68th Annual Golden Globe Awards held at the Beverly Hilton Hotel on January 16, 2011 (Photo by Vince Bucci/NBC/NBCU Photo Bank via Getty Images)
LOS ANGELES, CA - AUGUST 29: Actress Jennifer Westfeldt (L) and actor Jon Hamm arrive at the 62nd Annual Primetime Emmy Awards held at the Nokia Theatre L.A. Live on August 29, 2010 in Los Angeles, California. (Photo by Frazer Harrison/Getty Images)
(EXCLUSIVE, Premium Rates Apply) HOLLYWOOD - APRIL 22: Actress Jennifer Westfeldt and actor Jon Hamm attend US Weekly's Hot Hollywood 2009 held at My House on April 22, 2009 in Hollywood, California. (Photo by Jeff Kravitz/FilmMagic)
Jon Hamm and Jennifer Westfeldt during 1st Annual 'Show People' Tony Awards Party at Gotham Hall in New York City, New York, United States. (Photo by Djamilla Rosa Cochran/WireImage)
Jennifer Westfeldt and Jon Hamm (Photo by Bruce Glikas/FilmMagic)