It happens softly, gently. Never suddenly. She guesses that going insane must be a slow, gradual proccess, if she is indeed going crazy.
He never notices as she cries herself to sleep more and more often, so softly. She can't let him see her weakness, because it would(more) make her look insecure, it would make her look needy. It would make him worried, and she can't afford to be selfish. Can't keep him from living his dream if that is what's important.
So she says nothing. Says nothing as the tears flow freely from her eyes and fall softly to the pillow beneath her head. (less)