Persistence is a big part of success. Whether you're looking for a new job or trying to change careers, it's easy to find negative advice. The Internet is filled with experts who can give countless (bad) reasons why you will fail. However, if you're doing the right thing and you believe in yourself, success is almost always possible (See The Dip by Seth Godin).
Thomas Jefferson captured this idea in these words: "When you reach the end of your rope, tie a knot in it and hang on."