"Aspen didn't go as well as planned ... there were more lows than highs," explains a source. "Aspen was the final straw."
After spending time apart for their respective work commitments, Heidi and Seal had looked forward to hunkering down with their four kids over Christmas.
But "things had changed and they fought a lot," tattles another insider. "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."
The "Project Runway" mogul apparently became fed up with their bickering and decided it would be "better to separate before their arguing starts to affect their kids," adds the snitch.
But it seems the once PDA-disposed pair hid the friction from public view ...