"To put it very frank, after the show, we realized we weren't communicating as well as we once did," Colton, 28, wrote.
"Nobody was forcing us to talk about real things going on, so we let a lot of things build up."
Colton suggested he had initiated the breakup but Cassie agreed some separation would be best for them.
"In a weird way, I kind of broke up with myself," Colton shared. "I was like, I feel like we need to figure this out and in order to do that, we need space. She agreed. So it was a mutual thing."
Colton and Cassie committed to being in a relationship with each other -- without an engagement -- while filming The Bachelor in late 2018.
The finale of Colton's season aired on ABC in March 2019, and once he was able to go public with his romance, the former pro football player and Cassie led fans to believe they were madly in love with each other and came out of the show stronger than ever.
Things apparently came crashing down on the couple five months into their highly-publicized relationship, but Colton said it basically only took two days of them being apart to realize they were meant for each other.
"[Within two days] it was like, 'I miss you!' It was a very real and very emotional breakup, but it was also very good to be real and emotional because it forced us to have conversations and move on," Colton disclosed, according to People.
"Now, we know the flags to look for. When issues start to creep up, it's like, let's go have a conversation. We're trying to be more transparent and honest and really trust each other."
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Colton has said on numerous occasions he'd like to propose marriage to Cassie, but it seems she's been the one pushing back. Cassie has said there's no rush to wed or live with Colton and she's just enjoying dating and getting to know her man.
But on their future together, Colton wrote, "We talk and we plan and we have so much hope."
"If she were here, she'd nudge me for saying this, but I think an engagement is something we'd both like to see this year," Colton revealed.
"No matter what, I don't want my life without her. Our relationship hasn't always been easy, but it's been worth it."
Colton's book, The First Time, hits shelves on March 31. The book's title is apparently a nod to his reputation of being the virgin Bachelor.
Colton became a member of Bachelor Nation when he competed for Becca Kufrin's heart on The Bachelorette's fourteenth season in 2018. After failing to win her heart, he gave Bachelor in Paradise a shot but then went on to become The Bachelor star.
Colton had made up his mind before even finishing out his round of overnight Fantasy Suite dates.
Colton had therefore built up a lot of anticipation for his Fantasy Suite date with Cassie, and it seemed he was ready and willing to lose his virginity to her during their romantic evening together.
However, Cassie attempted to quit The Bachelor at that stage in the process because she didn't feel ready to get engaged after dating Colton for such a short period of time.
Colton nearly ended production because he was so devastated, but he fought for Cassie to stay and ultimately convinced her to give him another chance.
Cassie was hesitant to continue but decided to stick around for Colton now that pressure of an engagement was off the table, and then she proceeded to meet his family on the show and fall in love with him once filming wrapped.