“One of the most important qualities for young athletes is the ability to believe in oneself. If you have confidence in yourself, in your teammates and in your coach, you will succeed.” – Chuck Knox, NFL Coach
Confidence is essential for peak athletic performance; here is a great article discussing some mental strength strategies to build the confidence of elite athletes.
I wonder how this applies to self defence? My guess if you're not confident in your abilities to strike and then withdraw you'll avoid/ignore the best chances in the hope that the confrontation will bleed away to nothing, thereby potentially missing the chance to escape when you can.