Have you ever read about the cognitive behavioral concepts? These concepts are formed around understanding human cognitive distortions, unhelpful behaviors and emotional regulations that we form through our lives. It is but obvious, that every human being goes through many experiences through out their life and while going through them they learn about themselves and form beliefs about their own capabilities. Every belief that a person has of  him/her self is based on the past experiences. For example, if I have tried to play a sport a few times and have not been able to do well and in-turn have had bad experiences of facing associated humiliation, I would think a thousand times before I can try another sport. This is because I have formed an unhealthy opinion that I am not good at sports. On the other extreme, if I have baked a cake and everyone has loved it, I will have high level of confidence to try baking cake or cookies or anything else that I think needs similar skill