The fear of not being believed often comes from a place of abuse. A character who made themselves vulnerable to someone (a relative, a corporation, the media, etc.) and was then accused of dishonesty, unreliability, or worse—it's enough to turn that character into a bubbling mess of self-doubt and paranoia. There is nothing like an abuse of power to send one spiraling into a dark abyss that takes years to climb out of.

peaking up when there is zero doubt that what the character is saying is true
Being loose with the truth (because no one will believe them anyway)
Being honest to a fault—refusing to tell even a little white lie
Needing constant assurance that people believe them
Withdrawing from friends and family
Questioning everything that is said or presented to them
Being able to read others and recognize dishonesty
The character often accusing people of lying to them or to loved ones
Becoming angry if their word is questioned
Searching for the truth in all things
The character having nightmares about times when they weren't believed
Keeping quiet about abuse or unfairness (because speaking up won't do them any good)
Harboring a healthy distrust of other people, institutions, etc.
Extreme self-reliance from the belief that the character can trust no one but themselves

Wanting to speak up about an injustice but worrying they won't be believed