I have an ISFJ friend who is much the same way, but no amount of NTJ reasoning is soothing to him. I've found it's best just to listen and try to be as loving and accepting as possible. They'll work on the problem their own way eventually.
My mother is also an ISFJ, but a fairly optimistic one. Same thing applies when she's in a bad space mentally and emotionally, lots of affection (though it's not always easy for us Ts to give). Actually though, because I am typically not affectionate, very simple gestures (a hug out of nowhere, a card, a little gift) seem to really resonate with her (by which I mean she may or may not engage in frightening displays of crying and affection ). So yes, the positive reinforcement thing is a good one.