Because in spite of what she does or how she feels toward you and the M, she doesn't want her children to think she's bad.
I think that's part of it. I also think she doesn't want to be the bad guy in all this. It's avoiding shame. And avoiding her own loss of self-respect, I think. In W's words during C, I can "fall back on the excuse of depression". But I think she doesn't have an excuse, if you follow my meaning.
(formerly crushd) Married 14 yrs M41/W43/D7/S4 M: MLC, major depression/W: WAW Bomb 2/26/12, 2 days before anniversary Detachment, Grown Apart, "I love you like the father of my children", EA/PA?