There is a massive difference between someone who is competent and someone who is not.
Try talking with a customer service person who does not understand the product. Or getting a hair cut from a barber who hasn’t practiced enough.
When someone tells you adults are making it up, they’re trying to motivate you. But their message is, “fake it until you make it”.
Faking it until you make it not only feels bad; it also doesn’t work.
Instead, get competent. Develop your craft. Work persistently and consistently to get better.
It happens slowly, over time. But eventually, you’ll be competent, and the right people will recognize you for it.