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Keep in mind the brain's disposition to learn changes depending on age. And as an adult your brain will struggle for many different reasons: stressful day, lots of other information streams, far less time for learning, decreased flexibility, etc.

As a kid I learned 3 languages (including the native one) with relative ease. As an adult I really struggled to learn the fourth using what I think are the same techniques. Given the factors above the reality may be that it's almost impossible to learn in the same conditions.