I don't feel like "confidence" has much to do with it. In order to be successful, one must be willing to fail. As the saying goes, "The master has failed more times than the student has even tried". But if your every minor failure is met with overwhelming negative reinforcement then you learn that failure is bad and avoid it as much as possible.

I also kinda disagree with the "self-esteem" as a metric, but only because it seems to me that people try to solve that problem by telling people they're great and did good work even when they aren't and didn't. I think it is much more important for the message to be that failure isn't really a reflection of your worth as a person.