After that initial feeling, we can say and choose, "I will not fear. I choose to believe God will get us through this." Or, just say, "I choose to believe in God!" Or, "I will not fear! I will believe."
And then, you have to hold onto that belief as hard as you sometimes hold onto worry. Come on. We all are great worriers by nature. We can worry for hours and days on end on something that will probably never happen. Why not use that energy, that will to hold onto belief rather than worry? It's in the choice where we choose to obey or sin. Choosing fear is sin. Choosing to believe is obedience. Jesus says in John 14:1, "Let not your heart be troubled; believe in God, believe also in Me."
I want to share a quote from Eugene H. Peterson from his devotional bible. It is about choosing to live by faith. Here goes:
"Faith has to do with marrying the invisible and the visible. When we engage in an act of faith as Caleb did (see Numbers 14), we give up insisting that head knowledge is our primary means of understanding. Faith isn't a conclusion we come to simply on the basis of what we see. It's a conclusion we come to on the basis of who God is. And that, fusing of the visible with the invisible gives us a different way of looking at the world around us. It's often said that living by faith is a risky way to live. What isn't so often said is that living by sight has risks too. Even greater risks."