i have experienced shyness since i was young
the first time unable to talk to a girl i like at high school
but now i reflect on it, shyness is like a invisible mental wall you must climb and overcome to interact with other people
it is also an avoidance issue related to anxiety.
i think the only way to overcome it is exposure, and acknowledge that you fundamentally have this weakness.
Black Rose has hit it on the nail.
it is an emotional process of being unconfident.
to become confident we must feel like we can succeed.
so we must feel safe to fail.
Exposure won't work, if you don't know how to deal with the situation. Even if you know what to do, if you don't feel like you'd succeed, you won't do it.
So shy people need to be taught how to socialise, how to behave in different social situations. Like Black Rose pointed out, then need to practise these skills in safe environments where they won't have to worry about being humiliated, until they are skilled and experienced enough that they can be confident in their abilities, and then can perform them confidently in real life.
we do not feel safe to be vulnerable around others.
we could have been rejected too much as children.
True. Most children have to be told not to talk to strangers, because talking to people they don't know is natural to most kids, as usually it's a lack of fear that makes children open to talking to other people.
So in most cases, shy children were bullied or criticised a lot as children, and learned that talking to people was liable to lead to more suffering, and so avoid most social interactions.
So the main difficulty with shyness, is not actually that they need to learn how to be social, but rather, they need to un-learn what they were taught, i.e. that being sociable is harmful to them. It's much harder to un-learn something, to remove something from your memory and your brain, than to learn something.