people simply do not walk away from a relationship, especially one involving a child, without some sort of motivating agenda. Basically, the "I feel trapped, I need space, we aren't getting along, I'm not happy" blah, blah, blah, are surface-level excuses. What is the real reason? Tash suggested depression. Your implication that the house was already a wreck tends to lend credence to that. People going through a major depressive episode tend to have difficulty motivating themselves to do anything. If that's it, it is treatable but it will require a lot of patience and understanding on your part. Of course, there could be many other agendas that would not necessarily portend a favorable outcome, but hopefully that's not it.

My thoughts are with all three of you.