During their trip to Aspen, Colarado, the couple sadly realised their marriage was over.
"Aspen didn't go as well as planned ... there were more lows than highs," a source tells US publication People. "Aspen was the final straw".
"When they came together for Christmas as a family, things had changed and they fought a lot," another source tells the magazine.
"Their Aspen trip was a bit of a mess. Instead of enjoying being back together as a family, it was difficult for them to get along."
Apparently there came a point in the holiday when 38-year-old Heidi felt she was "done with arguing" and decided it was better to celebrate for the sake of the children.
The pair are parents to Henry, six, Johan, five, and daughter Lou, who is two. Leni is Heidi's seven-year-old daughter from a relationship with F1 boss Flavio Briatore.
Heidi and Seal, 48, announced their separation on Sunday, saying they had decided to go their separate ways after eight years together following "much soul searching".
The Kiss From A Rose singer told chat show host Ellen DeGeneres that he was "grieving" and that he had no intenion of taking his wedding ring off any time soon.
"Just because we have decided to separate doesn't necessarily mean you take off your ring and you're no longer connected to that person.
"We will be connected in many ways 'til the rest of our lives".