“Winners are not afraid of losing. But losers are. Failure is part of the process of success. People who avoid failure also avoid success.”

Rich Dad Poor Dad: What the Rich Teach Their Kids About Money-That the Poor and the Middle Class Do Not!

Source: Wooden: A Lifetime of Observations and Reflections On and Off the Court

No definite source has been found for this statement; though most often attributed to Sir Winston Churchill, and sometimes to Abraham Lincoln, it has only rarely been attributed to Campbell.
Disputed

The Wit and Wisdom of Alfredo Di Stéfano Kindle Location 98.

As quoted in Business Class : Etiquette Essentials for Success at Work (2005) by Jacqueline Whitmore, p. 25

It – How Churches and Leaders Can Get It and Keep It (2008, Zondervan)

Song lyrics, Bringing It All Back Home (1965), Love Minus Zero/No Limit

Source: The Subtle Art of Not Giving a F*ck (2016), Chapter 1, “Don’t Try” (p. 11)