Yolanda Foster's Health Crisis Put 'Intense Stress' on Marriage to David Foster

"Things really changed in their relationship as Yolanda got sick, and then when her health worsened," the source tells PEOPLE. "David travels all the time and they simply weren't spending any time together."
"David, 66, and Yolanda, 51, were wed at fairy tale wedding on Nov. 11, 2011. Yolanda was diagnosed with Lyme disease in 2012 and has spoken out about the challenges she's faced, including at one point losing the ability to read and write because of the illness."