EXCLUSIVE: Ben Affleck's 'Heartbreaking' Reaction to Jennifer Garner's Upcoming Wedding Revealed

Ben Affleck is said to be "heartbroken" as his ex-wife Jennifer Garner prepares to marry her longtime partner, John Miller, RadarOnline.com can reveal.

The 53-year-old actor, who only finalized his divorce from Jennifer Lopez earlier this year, is reportedly struggling to accept that Garner, also 53, has moved on for good with the man she now calls her soulmate.

The pair were married for 10 years after meeting on the set of Pearl Harbor in 2000, and share three children – Violet, 19, Seraphina, 16, and Samuel, 13.

Despite their divorce in 2015, Affleck and Garner have remained close co-parents, often photographed together at their children’s activities and family outings. Over the years, many hoped their warm friendship would rekindle into romance.

But now, with Garner set to marry Miller, 46, those hopes appear to have faded for good.

A friend of the couple claimed: "Jennifer's been making plans for the wedding without much fanfare, and she's the happiest she's been in a long time. She's been telling people that John is the person she was meant to be with. For Ben, that realization has been painful – he's always had feelings for her, and now he has to accept that she's really moved on."

Garner began dating Miller, a businessman and CEO of Cali Group, in 2018, three years after separating from Affleck.

Friends say her steady relationship with Miller helped her heal from her high-profile marriage.

A friend pal claimed: "John has been her anchor through everything. He's steady, thoughtful, and brings her a calm she didn't have in the past."

Affleck, meanwhile, has endured a turbulent few years. His two-year marriage to Lopez, 56, ended in March following months of speculation about marital strain.

Since the split, he's been spotted spending more time with Garner and their children, sparking renewed rumors of reconciliation – rumors that intensified after the former couple were seen laughing and embracing during a family paintball outing earlier this year.

Those close to Affleck say he has described his breakup with Garner as the "greatest regret" of his life. An insider claimed, "Ben has always felt, deep down, that Jennifer was the one for him.

"Now he realizes her life is moving a different way, and accepting that has been absolutely heartbreaking for him."

Garner has never spoken publicly about the engagement, though she was recently photographed in Los Angeles wearing a diamond ring.

In April, she was seen sharing an affectionate kiss with Miller, which friends interpreted as her way of confirming that the relationship was serious.

The actress has previously reflected on her marriage with Affleck with both love and candor. In a 2016 Vanity Fair interview, she called him "the love of my life" but admitted the relationship had been difficult.

"He's the most brilliant person in any room," she said. "But when his sun is shining elsewhere, it's cold. He can cast quite a shadow."

Sources say Garner has been honest with Affleck about her future plans. A friend claimed, "Jennifer was upfront with Ben and told him her future is with John. She still cares about Ben, but only in a family sense. He gets that – even though it's painful for him."

Despite his heartbreak, Affleck continues to praise Garner as an "amazing mom" and says co-parenting with her has been "wonderful." But those close to the actor admit that seeing another man step into his old family home has been a bitter pill to swallow.

The insider dished: "Ben understands that Jennifer deserves happiness. He's putting his energy into their kids and trying to find some comfort in the fact that she's at peace – even if that peace doesn't include him."