I've found that one of the the reasons why learning a second language at middle age is less effective, is because we already have a language we can express ourselves in with great mastery.
For children and teenagers, learning the intricacies and irregularities of a language is simply the only way they can learn to describe and apprehend the world --- and themselves in new ways and ideas, and not get shamed for saying something weird. Their quality of life is very highly correlated with how well they learn a language, whereas the same cannot be said for those who have already mastered one, or those who live in an environment that does not actively use a language they are learning (hence why learning French in school helps you so little, and why so few Japanese speakers are confident in their English).
Barring immersion, one way to force yourself to learn another language more effectively is to spend some time muting yourself in the languages you already know and allow yourself only to express your thoughts in the language you are learning.
For children and teenagers, learning the intricacies and irregularities of a language is simply the only way they can learn to describe and apprehend the world --- and themselves in new ways and ideas, and not get shamed for saying something weird. Their quality of life is very highly correlated with how well they learn a language, whereas the same cannot be said for those who have already mastered one, or those who live in an environment that does not actively use a language they are learning (hence why learning French in school helps you so little, and why so few Japanese speakers are confident in their English).
Barring immersion, one way to force yourself to learn another language more effectively is to spend some time muting yourself in the languages you already know and allow yourself only to express your thoughts in the language you are learning.