You couldn't be more correct when it comes to the human psyche. If you think you can you're right; if you think you can't you're right.
As a much more minor example than CTB, I was a distance runner and coached distance runners for many years. Many people come to the starting line with an excuse already tucked into their waistbands (figuratively speaking). They've got an excuse prepared, it's in their mind, just in case: "I was sick/I hurt my leg/it was too hot/it was too cold/the course was too hilly/the course was too flat", on & on & on.
If you've got a prepared excuse in the back of your mind (tucked into your waistband so to speak), when things get tough, when the race is at its most difficult point, when you're feeling your absolute worst, you will pull out that excuse and use it. Guaranteed.
You must come to the line with no excuses.