How to write a self-policing character

I want to write a character that was told constantly by their parents that their power is villainous/Monstrous (MHA universe). Because of this they are conscious of how they behave, act and express themselves, internalizing people's words and trying to be "normal".

The premise expresses them meeting someone who doesn't condemn them for their quirk and exploring that.

The question is. How to write the though process of that character in 3rd person limited perspective? Especially when they start out in the story as a young child (9 year old) and gradualy age? Any tips?