I don't really know if I'd call it Steve's biggest fear just because I don't think he is consciously afraid of it, but I do think that what shakes him up the most is betrayal. I don't just mean on a personal level, but also in the sense of his world/ideals betraying him. Anything that rattles his sense of self, purpose, and morality (which, as Captain America, are often one and the same) is dangerous. Kind of like how the basis for Nomad was Steve being thrown into despair because he couldn't reconcile his America-as-ideal with America-as-reality.

Trying to live as someone embodying an ideal is hard when you can't have steadfast faith in that ideal, y'know?