Is the fear of the unknown.
The unknown outcome, the unknown of what people will think, the unknown of whether it’ll be worth it.
The amateur never learns to control these feelings.
The professional learns to use them. Learns to use them to his advantage.
The professional knows they never go away. So instead of kicking them out and being angry, he invites them to dance.
When a new challenge arrives. The professional says, 'hello there.'
He knows it's going to be hard work. He gets his hands dirty.
And he reminds himself of the choice he made. The choice to spend time doing things that matter, known outcome or not. Because the most important things the professional does might not work.