Inside Yolanda & David Foster's Shocking Split: 'Distance and Her Illness' Drove Them Apart

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We're learning a little more about what led to Yolanda and David Foster's surprising split.

A source close to Yolanda tells ET that the couple ultimately decided to divorce after four years of marriage because "David is a very busy man, and always put work No. 1."

"He travels a lot for work and really did not scale back when Yolanda got ill," the source continues. "She’s also been sick for their entire marriage. She was diagnosed [with Lyme disease] right after their wedding so that really took a toll on their relationship."

WATCH: Yolanda Foster and David Foster Split

The source says it was David's busy schedule, pared with Yolanda's continuous battle with sickness that signaled the end of the couple's union.

"Her kids were always there when she needed them, but not her husband," the source says. "It was tough. Distance and her illness is what spread them apart."

Amid the split, Yolanda posted a somber photo to Instagram on Wednesday, writing, "There are blessings in every Day, find them, create them and treasure them....... #NoteToSelf."

WATCH: Yolanda Foster Hints at Marriage Troubles on 'RHOBH' Premiere, Co-Star Kyle Richards Talks Split

While announcing their split on Tuesday, Yolanda and David tried to remain positive about their time together.

"Sadly we have decided to go our separate ways," the ex-couple told ET in a joint statement. "We've shared nine beautiful and joyous years together. During that time we experienced love, friendship and the inevitable challenges that come with managing a marriage, careers, blended families and health issues."

WATCH: Yolanda Foster's Emotional Battle With Lyme Disease: 'Many Nights I Wished to Die'

Hours after ET confirmed the split, on the Real Housewives of Beverly Hills season premiere, Yolanda hinted at troubles in her marriage.

"He's so busy, finishing his big Disney project, his foundation. You know David, he's the man, he needs to work, he has a career," she explained. " That's part of David Foster, he travels, he does charities all over the world, it's been like that since the day I met him. That shouldn't change just because I'm sick."

WATCH: Yolanda Foster Sells Malibu Mansion to Concentrate on Battling Lyme Disease

"We are grateful for the years we've spent together and believe wholeheartedly that we did our best," they statement concluded. "We hope that we can pave the road ahead of us with all we've learned and with the love and respect we will always have for one another."

On her blog after the show, Yolanda added a heartbreaking account of how Lyme disease has in some ways derailed her life.

"I have gone from being an outspoken multi-tasking social butterfly to a hostage of my debilitating disease," she said. "I might only be a shell of the woman I used to be, but my heart is still the same, and even though I can only do life one day at a time, I am at peace and have surrendered to my new normal--for now."

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And in a possible reference to her marital troubles, the reality star's blog post also details how she has had to come to terms with "human nature," saying that "I've learned to accept people’s lack of long-term compassion for others while they live their busy lives."

Meanwhile, even as her life hits yet another hurdle, Yolanda's family continues to thrive. Watch the video below for Gigi Hadid's nude photo shoot on the same day that her mom and David announced their split.