It is actually irrational, but back in college I climbed a very tall extension ladder, stood on the top rung of it to wash a window. When I came back down, I had gained a fear of heights that over time has only gotten worse.
It might seem rational. I could fall and get badly hurt or even die, but I have even gotten butterflies looking at videos from high places. Now let's take a look at this rationally.
Take your greatest fear: water, heights, spiders, clowns, women with red hats or anything else you might fear. Now, what is the worst thing that could happen to you?
Let's say you have a fear of riding in a car. The worst thing that could happen is you get into an accident. What is the worst result of that accident? You die. What is the worst thing about dying?
If you are a Christian, the worst thing that could happen is you go to Heaven and be with God for eternity. Hmmmm. Not a bad result for your worst fear.
The worst result for me with heights is that I fall, die and go to Heaven. I admit that I still have that fear even knowing that the worst thing that could happen is to meet my Lord and Savior face to face, but there is hope even in my worst situation here in this world.
Trusting in God does not mean our human emotions, such as fear, are eliminated. However, it does mean we can go forward in spite of our fears, trusting Him as we follow Him.
"When I am afraid, I put my trust in you." Psalm 56:3