confidence vs. arrogance

Where is the line between these two mindsets? I often wonder about that. 
We, freelancers, spend most of our time in front of a PC and, as much as social networks help us get in touch with colleagues, we often stay alone with our thoughts. 
We are told, not only in the world of translation but in many areas, that to be successful in life we need to believe in ourselves. We must think positively, stand up for ourselves, carry on what we believe in. 
But then the question arises: to what extent do I believe in myself, and when does presumption take over?
It seems a simple question, but I have not yet found the answer.
We study, we inquire, we document. We talk to colleagues and confront them. Before sending a translation, we read it a thousand times, to make sure we have sent an impeccable work. But even when our efforts are rewarded, something always leaves us unsatisfied. Have I done enough? Could I do more? We always have to confront ourselves and learn to live with the many aspects of our personalities.
What do you think?