Imagine if someone, maybe a parent, said, "I doubt you'll ever amount to anything." Those words cause you to doubt your potential. So you overwork yourself, trying to get perfect grades in school. And you start stressing out and becoming more and more anxious as you doubt your potential. This develops into you having a hard time connecting with other people because your mind goes blank every time you try to talk to someone. You stay up late watching videos on how to be a good friend, how to make friends. It's this doubt in your potential that keeps you lying in bed, worrying all night. And it's also doubt that keeps you from ever improving and escaping your hopeless situation.
As I understand it, that's what doubt-ing is. The idea is that the elements are something to be expressed and illustrated. If doubt causes you to run away from home, then running away from home is an act of doubt-ing. If doubt causes you struggle speaking to other people, then, in this context, struggling to speak with others is doubt-ing.
The doubts themselves are something static on their own, but "doubt-ing" is the process of these doubts actively expressing themselves and being illustrated within the story.
This process is an opportunity to show why the doubts are good or bad, and why these specific doubts are a problem or a solution within the context of this story. If doubting myself causes me to run away from home, but that's what saves me from a zombie apocalypse... maybe doubting my potential is a good thing in this story? Maybe a critical moment is when I choose to NOT take a leap of faith over the Grand Canyon, because unlike everyone else, I know I'm literally incapable of jumping over the Grand Canyon. So maybe I need to remain doubt or disbelief in my own capabilities if I want to stay alive. Maybe the doubts I described in the first paragraph are the backstory for why my character views doubt to be a bad thing (it made them nervous and unable to connect with others), and the story leads to them realizing that in some contexts, it's okay and even healthy to doubt yourself (expressed through choosing to not attempt jumping over the Grand Canyon).
Basically, my main point is that if you have an element, let it be expressed. And preferably, illustrate it in ways beyond a character simply saying "I'm so doubtful. I don't think I can do this." Have them travel across the multiverse, have them dance, have them sing, have them murder, have them smell like pizza, and so on. Those are all valid forms of doubt-ing if you allow them to be.