Learning New Languages - What's So Hard About It?

Most advices for learning new languages are about "a lot of practice". Uh, sounds reasonable! Then what? Many people still have difficulty learning languages. Why is that? "Practice" is rather a vague term. Though I'm no expert, I'd like to share my thought on the topic as someone who has learnt 2 foreign languages quite well.

First is an article I ran into yesterday on wikipedia - Sapir-Whorf hypothesis
These are some excerpts from the article:
In 1820 Wilhelm von Humboldt connected the study of language to the national romanticist program by proposing the view that language is the very fabric of thought, that is that thoughts are produced as a kind of inner dialog using the same grammar as the thinker's native language.
"No two languages are ever sufficiently similar to be considered as representing the same social reality. The worlds in which different societies live are distinct worlds, not merely the same world with different labels attached." - Edward Sapir
... Whorf was not principally concerned with translatability, but rather with how the habitual use of language influences habitual behavior. Whorf's point was that while English speakers may be able to understand how a Hopi speaker thinks, they are not actually able to think in that way.
Eric Lenneberg's formulation of the hypothesis:
1. Structural differences between language systems will, in general, be paralleled by nonlinguistic cognitive differences, of an unspecified sort, in the native speakers of the language.
2. The structure of anyone's native language strongly influences or fully determines the worldview he will acquire as he learns the language.

There is a stress on "native language". It seems to me this is because it's assumed that the person's proficiency in his non-native (second, third, ...) languages is often (much) lower than in his native. Furthermore there is a speculation that it's impossible for speakers of one language to think the way of another language's speakers. We know the best language learners are children, who don't have a fixed way of thinking or a fixed worldview (yet). I would say this is the root cause of adults' difficulty in learning a new language.

However, I disagree with the assertion of impossibility. I myself has learnt 2 languages rather successfully not by practicing a lot, but by forcing myself to think in those languages, using the "kind of inner dialog using the same grammar" mentioned by Humboldt. I think this is very efficient, especially when the new language is immensely different from the native one (in my case the highly synthetic Russian versus the highly analytic Vietnamese).

So, what's the point of this? It is - a powerful way to learn new languages is to relax the existing worldview to think, really think in the new language. It is the highest form of language immersion. I suspect that it was the way Mezzofanti "fluently spoke thirty-eight languages and forty dialects, despite never traveling outside of Italy."  And let me stress this: translation to one's native language is extremely harmful for learning new languages.

How about the "language superiority", and are computer languages related to this (say, Turing-complete and Universalist theory of language)? That would be another post.