A "big thing" for the couple was the fact "they had issues with their communication styles," The Sunreported.
"Katie is much quicker to call little things out whereas Blake just lets things slide," the source said.
"Her outbursts eventually became too much for him to handle."
Katie, 30, and Blake, 30, revealed in a mutual statement earlier this week they had decided to go their separate ways "with mutual love and respect" because they are "not compatible as life partners."
The source told The Sun, "At the same time, Katie felt she was putting more into the relationship than he was."
Since announcing her breakup, Katie has revealed on Instagram Stories she's struggling to mend her broken heart.
While Katie shared how it's been difficult for her to leave the house, she wrote on Wednesday how she plans to "accept what is and learn and grow from it."
Katie and the wildlife manager reportedly broke up this past weekend after the former bank marketing manager performed stand-up comedy for the first time in front of an audience at a show in Anaheim, CA on Friday, October 22.
"Katie flew up to Canada early Saturday [October 23] just a few hours after the comedy show and saw Blake for the first time in a while," the source explained.
"I don't think she flew there necessarily knowing it would be a breakup trip, but she knew they'd be forced to address a lot of things that they had been putting off for a while."
Multiple reports have circulated that living in different countries is a major reason why Katie and Blake didn't work out.
"Distance was a big factor," the source noted.
Katie just moved into a new San Diego, CA, apartment in August, and Blake -- who lives in Toronto, Canada -- often travels to Africa for his endangered-animal conservation efforts, most recently involving the rescuing of rhinos.
Katie reportedly missed Blake when he was gone and wished he could be with her more.
"Blake wasn't ready to leave Canada and Katie wasn't ready to move there," the source explained. "She just set down new roots in San Diego and loves it there."
The source reasoned, "So it's a number of issues and not one person's fault. They were just clearly not compatible and rushed into things."
However, the source insisted "infidelity was not a factor" in the pair's split.
Although Katie hoped Blake would be her person forever, an insider recently told Us Weekly that she "won't give up on love and still hopes to get married one day."
When Katie and Blake released their joint breakup statement on social media, they called themselves "grateful" for the moments they had shared but said it's "the most caring choice" "to move forward independently."
During the interview, Katie -- who originally competed for Matt James' heart on The Bachelor's 25th season -- had sounded somewhat indifferent about her relationship with Blake, who first appeared on The Bachelorette's sixteenth season last year and joined Katie's season several weeks into the process.
"We're figuring it out," Katie had said. "We don't know. This is very new to us still. I'm just focusing on myself, he's focusing on him, and if we work doing that, great."
The couple's breakup may have shocked many fans because two weeks ago, Katie had taken to Twitter and clapped back at speculation she and Blake had called it quits.
On October 15, Katie tweeted a screenshot of one of her recent Instagram comments that read, "You and Blake are not liking each other's posts. Is all OK?"
The screenshot showed that Katie immediately responded to the fan, "Yes," with a tears-of-joy emoji.
Katie tweeted alongside the image, "Imagine your fiance has been on his [Instagram] stories for weeks in Kenya working his ass off and yet this is what some people notice," in addition to another tears-of-joy emoji.
But when Blake left for Africa to focus on saving animals, Katie reportedly wrote on Instagram earlier this month, "It can feel lonely moving to a new state... I've had trouble sleeping lately... I'm still trying to navigate this new life and city while seeking normalcy."
Katie and Blake had said they were in no rush to wed and intended to travel together and learn more about each other's backgrounds, interests, hobbies and hometowns before deciding on an "anchor spot," or city in which to put down roots as a couple.
"I don't think people fully understand what it's like to date someone from another country and the rules behind that and the visas behind that," Katie said in a series of late-August videos posted to her Instagram Stories.
"I think right now Blake can only be here for 60 days before having to go back to Canada. So we really do have a lot we have to look into -- and we just haven't yet."