Have you ever seen a father stand his young child on a wall, then step back and open his arms? Without hesitation, the child jumps off the wall, and the father catches them. They squeal with the scariness and the joy of it.
Why do they jump so readily? Because they know and trust their father that if he asks them to do it, it will be safe. He will always catch them and never let them fall.
When God calls us to do something, why are we so scared? Because, unlike the child, we don’t know the Father well enough to trust Him.