The overall lesson is that thinking less positively about yourself and your capabilities creates a loss of self-confidence. So, in order to gain self-confidence, rather than continue losing it, just do the opposite.
Indeed, this is the entire secret to regaining your self-confidence: thinking more positively about yourself and your capabilities.
Or, as people commonly put it: believe in yourself more. (Which is something you'll learn more about very soon.)
You don't have to exercise, lose weight, make a lot of money, wear fancy clothes, or look like a supermodel to feel more self-confident. After all, having more confidence in yourself is simply a matter of thinking more positively about yourself and your capabilities.