There is a saying that “you can’t know what you don’t know”. This might be tautological, but it is also true, as it might be impossible to identify gaps in our own knowledge. In other words, you can’t teach yourself what you don’t know. Without instruction and training, you’re very likely to think that you do in fact know “everything” you need to know, when in fact, you don’t have the ability to recognise your mistakes.
Typically, the unskilled rate their ability as above average, much higher than it actually is, while the highly skilled underrate their abilities. Confidence is no substitute for skill and knowledge, while skill and knowledge must be used with confidence to ensure a positive outcome.