How did you successfully become fluent in another language?

My name is not your business. When i was 13 years old, i determined to learn the French language. In less than a year, i was sufficiently fluent in reading French that i could comfortably read novels. Eventually i became almost aa fluent in reading the language as i am in my native language. However, when i arrived at university, i learned that my high school French teacher (whose course i had taken for two years) had no clue about the pronunciation of the language. I was young, and mortified about my clumsy spoken French, so i only took the courses i needed to complete for the degree program.

However, a decade later i got a job working with francophones. One of them was my supervisor, and he didn't like to speak English if he didn't have to--so he spoke only French to me. He was patient, and that was necessary for me to learn. I also became friends with many Africans for whom their first European language was French. So i had another source to learn the language in a casual, social setting.

I found that reading modern plays in French gave me a good sense of the rhythm of the language, as well. Essentially, teaching myself to read the language had proven relatively simple. That made me fairly confident when i subsequently took course in French in high school. However, when i arrived at university to learn that my spoken French was hopeless, it sapped all of my confidence. It was only when in was in an immersion setting, with people who were patient and tolerant that i learned to speak French tolerably well.

I suggest that you study the concept of communicative competence in langauge learning, if you haven't already.

I successfully became fluent in the English language because I had to. Emigrating to the USA at age 11, at home we spoke only Latvian. I had some basic kid's familiarity with German as well, but was not fully fluent in it, not even in a childish way. I picked up English by attending public school where nobody was likely to speak any other language with me. I still speak Latvian fluently and have done interpretation and some translation work on a contract basis with the US Dept. of State. I've also picked up some conversational Spanish and French along the way, mainly simply by being exposed to it. And I've kept up with my German; I'm still not quite fluent but can get by without making a fool of myself, thank you.