Might you have depression, OP? PPD? I have intermittent depression, and this seems to be a symptom. Off and on, I am consumed by saddening thoughts--I just can't stop thinking about them!!--and sometimes those things suddenly don't bother me so much, even though I still think they're sad. If you have a more consistent type of depression, you might not have those relieving moments of "eh, whatever" between the depressive episodes. One thing I've noticed helping is to become sad enough to actually start crying. I feel better afterward. I guess because it gets rid of cortisol. So the actual tears are the important part; make sure you're nice and hydrated!
...Where do people get this "parenting doesn't matter" idea from? Doesn't that fly in the face of every study, research, book, anecdote, personal experience, common sense, etc? Even that one article that tried to say parenting didn't matter was only talking about variations within a certain category of parenting ("vaguely normal families in the First World," whatever that means) and only accounted for certain similarities in the resulting adults. I think most kids look happy when they're playing over at a friend's house, even if they're horrifically abused, so I don't know how that's any kind of gage of parenting competence.