"Jen and John called it quits several months ago. They've remained friends. They just started seeing less and less of each other and finally called it quits," the source explains. "She has nothing negative to say about John; they just seemed to drift apart. She says he was an amazing stepping stone in her life."
"Jen's focus still remains on her children, her career and spending time with those who make her really happy," ET's source adds.
"He is warm, fun and incredibly smart," the source said of Miller, who's the CEO of Cali Group -- a tech company that owns Miso Robotics and a chain of 50 CaliBurger restaurants -- and has also been married previously. "He is a real success in the business world and has no interest in being in the entertainment industry."
In January 2019, a source told ET that Garner was getting "serious" with Miller, who is himself a dad of two children from his previous marriage.
"Jen feels so lucky to have met John and their relationship only continues to grow," the source said. "They both wanted to take it slow, but it has gotten serious and they couldn't be happier. John's been at their Palisades house so much lately. They all [John, Jen and their kids] stay up late playing board games."
"Jen has something very different with John than she had with Ben," the source added. "She and John are both interested in living a healthy lifestyle, working out and eating well, except for the occasional trip to Chick-fil-A! When they aren't with the kids, they take time on their own seeing movies and having romantic dinners. They've already taken the kids to the mountains together, and Jen's friends describe John as a mountain man. He loves to do outdoorsy things."
Last May, a source told ET that the couple was spending more time together after deciding to "slow things down a bit" in December 2018. According to the source, Garner took the break to focus on her kids, her career and co-parenting with 47-year-old Affleck.
"They have great chemistry and truly love each other's company," the source said of Garner and Miller. "In the last couple of months, while they have taken a little time for themselves, they have missed each other and both realized they wanted to spend more time together."
"Jen wants to continue to explore their relationship and see if John is the right one for her," the source continued. "John adores her and he is comfortable with her celebrity status, which is a relief for her. Jen's friends are encouraging her to spend more time with John. They think he seems to be the obvious next step in her life."