I was exactly wondering whether you just learned that language a few years ago and you'd somehow mastered it to this level in a short time or if there were some circumstances that would explain your proficiency better. It's quite rare to find anybody that spends even a moderate time in Japan to know the language at that level.
A good friend has her degree in Japanese and she would not even begin to claim that she's either fluent or comfortable in the language, and that's after 5 years of studying it intensely. To her it's a 'good basis to start learning Japanese', which she says is almost impossible outside of living in Japan for an extended period. It's against that background that I'm impressed, I know a little bit of how much effort that must have been, even if plenty of it went 'as you go' by immersion.
I think that I was looking for exactly the first two words of your reply.
My own 'second mother tongue' (English) I've been learning since I was 10 years old or so and I'm still learning, there isn't a day that I find some turn of phrase that I hadn't encountered before. I've also noticed that speaking a few languages helps to make you (much) more forgiving towards others in a foreign tongue.
Do you come across many foreigners in Japan that have your level ?
(If you think the bit about learning Japanese can be compartmentalized away from the bit about learning to persuade or learning what big megacorps value in new products, I respectfully disagree, but you'd probably find the answer "3 years college, 1 year study abroad, 3 years professional experience in (mostly) J->E IT translation, 2 years in software development/management for aforementioned megacorp" more useful.)