Sherlock Holmes said "my mind rebels at stagnation," and I couldn't describe myself better than that.
In the Summer of 2001 I was on a 3 month break from university and I had nothing to do, so I pretty much just picked up my brother's old German high school textbook and started reading. Eventually I collected more books, and a dictionary. I bought a couple of computer programs but they were pretty useless. I focused only on reading at first. Eventually, I bought some proper German storybooks and after about 2 or so years, I started reading Harry Potter. I chose HP because I'd already read them in English, so at least I knew the basic story. It seemed impossible at first, but bit by bit, day by day, it started getting easier. By the end of the fourth book, I was reading through them at about 90% of my English reading speed and very rarely having to consult a dictionary.
This was back in the days of Microsoft messenger, so when I was brave enough, I logged onto the random chatrooms and just joined in conversations. Often when I slipped up and was forced to admit I was still learning, everyone jumped on me and wanted to chat. Germans are very happy when English speakers learn their language. I guess it's usually always the other way around for them. I met friends on those chatrooms that I'd later meet up with in Germany and hang out.
After 3 and a half years of that, I went to study in German for 6 months. My big moment had finally come. Mind you, I hadn't spoken a word of German to anyone at that point, but by the time I left, I had two and a half hour long conversation with a business man sitting next to me on a flight to Frankfurt, and he told me halfway through that he had no idea I wasn't German. Him telling me that was like a dream come true, four years in the making.
I guess you just have to really want it, and you have to put the time in. I mean, lunch breaks, during your commute, after work. You have to do it every day. Play to your strengths. I'm a visual learner, so I put post-its up on my wall next to my bed with verbs and articles and conjugations that I wouldn't take down until I'd memorised them front and back. But some people learn better orally, or by memorising grammar rules.
It has to become a passion and a challenge. A bit like a hobby really. Even in the darkest moments, when I'd realise how little I actually knew, despite putting years into it, I never entertained the thought of giving up on it.
Problem is, I very naively thought that once you'd learned something, it would just sit there in your brain, like books on a shelf you could access anytime you want. I made the same mistake again when I stopped playing piano regularly.