My native language is Estonian, as I am, in fact, an Estonian, but I'm in the same predicament as you. I used to write stories and stuff in my native language when I was younger, even winning some writing contests, at my school, and outside of it (of course, I was a small child, and we wee creatures have much lower standards), but when I started frequenting the internet, the lingua franca here, English, was what I suddenly started to actively use and advance.
Of course, my English has always been good. In comparison to what generally others my age had/have, at least. When I was still in kindergarten, I watched a lot of Cartoon Network, and knew some basic English before even going to school. Starting English from first class also helped. All the time I was all over my native language, I was balancing it with a growing interest in English, and, well, the internet was what finally swayed me over to one of the sides.
Through the years I've used English more than my native language (going purely by a words-read/heard-to-words-written/spoken ratio, seeing as people outside have physical activities and emotions present there to fill in gaps between conversation, thus enabling to give peoples' blabbering some resting time), and my native language I've mainly used just to exercise casual chatter, going beyond that only at school, in lessons, knowing a bit more technical terms than in English (through the help of chemistry, physics and the like). Otherwise, my conversations with others are pretty laconic (due to my shrinking Estonian vocabulary) and simply worded, while, on the other hand, I always try to increase my vocabulary in English, and all of the fiction I write nowadays (recently one certain big "project" of sorts), including anything non-fiction (unless it's for a school assignment, excluding for English class) is in English, where I consistently try to improve my abilities in wordsmanship.
And after that dreadfully long shit, I'll finally come back to the topic of the thread. Due to everything I described above, my Estonian is gradually getting worse, especially seeing as with every day that passes, I use more and more anglicisms in my speech. I don't know, I just like the language more than my own (to which the fact that it's so widely used world-wide may contribute).
But seeing as I plan to move (maybe temporarily, maybe permanently, y'never know) to an English-speaking country (and I'm pretty sure which one), I don't really worry about it too much.
tl;dr: My linguistic abilities with my native language are going down the shitter, while I always try to improve my English, due to which anglicisms have started to litter my speech like flies over a most extraordinarily juicy and moist piece of fecal matter. So...high-five, OP?