I moved to Italy in 2006 without knowing any Italian (apart from “ciao”, “birra” and “grazie”). I toke an intensive course of 4 hours a day from Monday to Friday for 3 months and, when I left class, I tried not to speak other languages, although my Italian was painful at the beginning, I used it at all times and with everyone, even with foreigners.
Shortly after arriving in Italy, I started dating an Italian, which helped me a lot to improve my accent (even now some Italians ask me if I am from Florence). I lived there for a year and when I returned to Barcelona, my ragazzo came with me, so I continued to use Italian at home for a couple more years.
I did not learn Italian in 3 months, that was the time I invested in my classes. But I did many hours of practice. In fact, I did so many that I even forgot my other languages! When I returned to Spain, I was unable to speak Spanish, I had neglected it for a year and it took me a month to regain my confidence and fluency.
As I always say, languages have to be practiced regularly to be able to use them without having to think, even if it is your native language. The only way to learn fast is by spending hours. The more you practice it, the more fluent you will be and the more comfortable you will feel speaking it.