It's wrong to doubt yourself.
Jacob, who has low self esteem, listens to his boss offer him a promotion, but the promotion has demands above his skill level.
Jacob wants to make money.
Jacob takes classes to sharpen his skills, and asks his coworkers to help him out in his new job.
Jacob refuses the job choosing to stay at his current position.
Jacob has higher self esteem because he's excelling at something challenging.
Jacob continues working at a dead end job.