As kids, we get them from our parents and draw our childhood conclusions, based on what our teachers say and the glances our classmates cast at us. Then we grow out of these beliefs, like we do out of our children's shoes. But even when they slow down our growth and motion, like awfully tight sneakers, we rarely think that things could be different.
While we still have them – the "All competitions are corrupted", "It's too late to have a child at 30", "You can't combine work and family", "I'm not good enough" – the world will keep giving us the proof.
While we have beliefs about ourselves, like "I don't deserve to win a major competition", "My talent is not realized", "All people are assholes", etc – everything that happens to us will only prove this subjective inner truth that we have.
It's your own choice, what to think of the world and yourself.