AceShowbiz - John Cena and Nikki Bella shocked fans after they announced their split after 6 years together. And now, the alleged reason behind their breakup is revealed with a source suggesting that John had "cold feet" just three weeks before their wedding, which was initially planned to be held on May 5.

"[John] talked a really good game about having changed, about having his priorities straight, about knowing what's important. And that it was [Nikki]," a source tells PEOPLE of the 40-year-old wrestler. "But as the day got closer, it was like he just went back to who he's always been -- which is someone who puts himself first, always."

"He's an incredibly dedicated, driven, ambitious guy. And for the longest time he said he didn’t think he had room in his life for a spouse or kids. It seems like in the end that was still the truth," the source continues.

John himself made it clear that he didn't seem to see having kids in his future. "I've told you I don't want to get married and I don't want to have kids, and you do want to get married and you do want to have kids," he told Nikki in a 2014 episode of "Total Divas". "I feel like there's a time bomb over my head."

John, however, seemed to have a change of heart as he popped the question to Nikki last April in front of 70.000 fans. But now that he called his engagement to Nikki off because he didn't seem to be fully able to change his opinion on having kids, the source says that "he never should've proposed, let alone in front of million of people."

"Now she's heartbroken," says the source of 34-year-old Nikki. "This was supposed to be the happiest time of her life. He ruined it and blew everything up."

"While this decision was a difficult one, we continue to have a deal of love and respect for one another," the former couple announced their split in a joint statement on Sunday, April 15. "We ask that you respect our privacy during this time in our lives."