In the book, The War of Art, author Steven Pressfield talked about what separates amateurs from professionals and how a professional became who they are.
I will write about what these aspects mean to me personally.
The professional acts in the face of fear
The pro recognizes the fear is always there – it is part of him and always will be. The effort spent on conquering fear is a waste. Rather, the pro learns to live with his fear and prevent it from turning into paralysis. [http://sethgodin.typepad.com/seths_blog/2014/04/how-do-i-get-rid-of-the-fear.html]
The professional accepts no excuses
The pro does not rationalize an excuse, or worse, believes in the rationalization. He knows the excuse is a lie he tells himself. If he accepted the excuse, the lie will perpetuate. He chooses to continue working rather than making up excuses.
The professional plays it as it lays
The pro understands the playing field is never leveled. So many things can happen to cause the field tilt one angle to another, day after day. The pro just learns to exceed his best with what was given.