Self-confidence is about balance. At one extreme, we have people with low self-confidence. At the other end, we have people who may be over-confident.
Good self-confidence is a matter of having the right amount of confidence, founded in reality and on your true ability. With the right amount of self-confidence, you will take informed risks, stretch yourself (but not beyond your abilities) and try hard. By contrast, if you are under-confident, you’ll avoid taking risks and stretching yourself; and you might not try at all. This means that you’ll fail to reach your potential. And if you’re over-confident, you’ll probably take too much risk, stretch yourself beyond your capabilities, and crash badly. You may also find that you’re so optimistic, that you don’t try hard enough to truly succeed.
So, self-confidence needs to be founded on reality: realistic expectations, your skills and experience, and the effort and preparation that you are willing to put in to reach your goal.