To harness something denotes that it already exists. That it doesn't have to be created but instead that thing merely needs to be discovered & directed.
You already have power, probably significantly more than You know. Our jobs are never to create power - it already exists within us - but to gather it, to combine it with love & learn to use it in order to shine Our inherent light. And step one in this process is to fully believe in it's existence, to faithfully believe in Self.
Here's an exercise to combat self doubt.
First find a spot on Your body that is easily accessed by You but would have to be touched on purpose. Something like gently pinching Your left earlobe. Whatever spot You choose, remember it. Now onto step two.
Recall a time where You had victory over something in Your past that seemed insurmountable, could have been learning to rid a bike as a child or finishing the school year (remember how long that year seemed when that first test in September rolled around!). Whatever the victorious memory, close Your eyes & visualize it. See Yourself in that moment of triumph over great odds. Feel how proud You were in that moment, how strong You were in that moment. Recall all You went through, how You conquered self doubt and all other challenges internal & external to reach that win. As You feel that pride in Your preparation, dedication, perseverance and triumph find Your spot (remember the example of pinching that left earlobe) and, this is very important, while feeling all those feelings of triumph during Your visualization "place" those feelings into that spot by (using my example) pinching the left earlobe. Pour them into that spot. Feel the awesomeness & confidence of that experience pouring into Your "spot" (left earlobe) as You touch it.
Now repeat with another powerful, victorious memory.
What You are doing is creating a physical point of power, essentially anchoring those feelings of awesome into a place where You can readily retrieve them when You need them. Like most things the more You practice and repeat this exercise the better able You will be in recalling the power of Your victories in moments where fear & self doubt present themselves.