Jennifer Lopez and Ben Affleck's Rekindled Romance 'Is Not a Summer Fling': Source

Jennifer Lopez and Ben Affleck's romance won't fade away with the season.

A source tells PEOPLE the couple is "very much enjoying their vacation" touring the Mediterranean islands to celebrate Lopez's 52nd birthday.

"This is not a summer fling. They are planning a future together," says the source. "They have serious talks about how they can make it all work. Between their relationship, their kids and jobs, they know it will be tricky to balance it all."

"Jennifer is the happiest," adds the source. "She keeps gushing about how amazing Ben is."

Affleck, 48, and Lopez began seeing each other again in April after their respective splits from Ana de Armas and Alex Rodriguez earlier this year.

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The couple originally began dating in July 2002 after filming Gigli together. They got engaged that November before postponing their September 2003 wedding, just days before the date, and ultimately called off their engagement in January 2004.

Ben Affleck - Jennifer Lopez
Ben Affleck - Jennifer Lopez

Emma McIntyre/Getty (2) Ben Affleck (L); Jennifer Lopez

Lopez and Affleck made their new romance Instagram official on Saturday when they shared a passionate kiss in a photo she posted.

"5 2 … what it do …" she captioned some photos of them celebrating her birthday aboard the yacht. Although her pal Leah Remini previously posted a photo of them at her birthday party, it was the first time the lovebirds shared a glimpse at their relationship on their own social media accounts.

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A source recently told PEOPLE that the pair "are madly in love" and "the loves of each other's lives."

Another source close to Affleck and Lopez noted that the Justice League actor is "very happy" with the singer, and that "she's wonderful for him," while an insider close to Lopez added, "They want to do everything they can to make this work."