I speak two other languages fluently, and I'm a slightly different person in each language. It's really fascinating.
It takes a long time, I find, to reach a comfort level in another language that rivals that of how you express yourself in your mother tongue. I've been speaking Portuguese (my 3rd language) at least 50% of the time for the past 7 years or so and I'm still not able to hit my mark, as if it were my own...but I'm close. Btw, these last few years, I speak it 80% of the time.
If I were to venture a guess as to why 'hitting my mark' isn't so easy, I'd say that with most language learners, there's a certain point in your learning where you say to yourself, "I'm fluent enough". Reading and writing, I'm in the 95 percentile in Portuguese but with speaking (seemingly no matter how much I speak), I stay at around 85%.
That, and a new, huge, appreciation for anyone that learns the easy-to-start, nightmare-to-master, English as a second language.