It always comes a time when an illustrator has to take decisions, push the tip of his pen on the paper and dirty up the white space in front of him. If it is a complicated line he is facing, he will feel kind of stuck, afraid of ruin his work, freezed thinking about that next line over and over. That is when he will get to understand that the sign he is adding to his drawing is like a leap into the void. There is no way back.
In my experience I got to understand that drawing was a training for real life, where you can prepare your choices, arrange your next move, pay the more attention you can on every detail, wait for the best opportunity, but in the end there will come a time where you have to jump, and try something you cannot fully foresee.