Dan Pink's book in a 10 minute speech on motivation. It will shock you what you've been doing wrong all these years. But don't worry, it's easy to fix. Also see http://masterytv.com/why-do-people-quit/
This true story of hope, tragedy, and inspiration at the Barcelona Olympics in 1992 will leave you in tears. You'll get up and get back to that thing you quit and you'll go help someone who has decided not to quit.