In some cases, fear of looking dumb is a symptom of social anxiety disorder (APA, 2022), and it can be associated with perfectionism and fear of failure. It can show up in issues such as imposter syndrome, or feeling like a fraud and worrying about not rising to the expectations of a high-achieving position. It can also be related to stereotype threat, when someone's membership in a marginalized group leads them to worry that they will act in a way that confirms negative stereotypes.
Tunnel vision happens when your mind zooms in on a single "threat cue" and filters out everything else. In this case, the threat cue might be: "He was young." "It was cancer." "It seemed sudden." "He probably didn't see it coming." Your mind grabs onto these details and begins building a narrative: "Cancer is everywhere." "People are dying young all the time." "It's inevitable that I'll get something serious." "If I do get sick, there will be nothing I can do."