Fëanor might have softened at the words of
Nerdanel until she mentions
Fingolfin, and he stiffens at that. He genuinely doesn't think he has acted wrongly in his conflicts with his half-brother-- he realizes he made an inaccurate judgment about
Findekano, but he believes he has good reason to be suspicious of Nolofinwë himself, and that Nolofinwë was wrong to lie to him about the whereabouts of Fëanoriel and Nelyo.
But
Maitimo speaks up before Fëanor can say anything. "Mother... don't worry about that. It is not Father who has led me to be more suspicious of danger than I used to be, but the brothers and the deeds they did. I fear danger, it's true, but that is not because of the words of Father, but because of... what has happened."
He can read the worry that lingers in her eyes, though, and tries to reassure her. "I'm not afraid of
everyone. There are those I still deeply trust."
He doesn't mention, though, that neither she nor Fëanor are completely on that short list. Not that he thinks they mean him harm, but rather that they have failed to handle the danger properly and in fact have increased his suffering, if unintentionally. Perhaps Ancalirë and the Nolofinwians are the only people left that Maitimo has complete trust in-- but even then, they are not capable of protecting him from all harm, which is why he is determined to be constantly armed, more skilled at fighting, and more vigilant.
It is ironic that he's trying to reassure Nerdanel that his own thoughts are not as dark as they really are, when he's
the one who's forced to live with them.