[DiogenesKynikos]

Children spend pretty much every waking hour - every day - learning language. If you were to put in that amount of constant effort, you might also learn language just as effectively as a child. Okay, probably not just as effectively, but I think people underestimate the amount of effort children put into learning language. That's practically their job for the first 5+ years of life.

The main reason why people fail to learn languages is that they do not put in enough time. There is no magic shortcut, despite countless language-learning programs claiming they have one. You have to spend a significant amount of time every day working at it.

Having good resources (e.g., access to native speakers, competent instruction, a flashcard app like Anki) is important, but again, people fail mostly because they don't dedicate enough time towards the language.