How are your characters tried and tested? Do they pass the tests you set them? In almost any story you can think of, it is only when the characters are put right through the emotional wringer, do you see what they are capable of and where their limits are. That is also where sidekick characters are useful - to help see your leads through to the bitter end and to get them through that "about to give it all up" moment.
Testing doesn't need to to be overly dramatic either. It can be as simple as a character having their patience tested by a relative who is trying to goad them (deliberately or otherwise, it's not always consciously done). Your character is very patient to begin with, then they become slightly less patient as time and the goading continues, and so on before they snap or do something they would not ordinarily have done. Your character may be aware of this process so what do they do to try and fight it? To not give in to that urge to snap no matter what the provocation?
Think about what would stress your characters out. Think about their coping mechanisms. What would happen if they failed or weren't available when needed?
Have fun finding out!