It's not as uncommon a problem as you may think. Actually, I'm exactly in the same boat.

It's been years since I wrote a piece of fiction in my native language, Italian. If I did that today it'd feel totally foreign. Rhythm and sentence structure tend to be different, I've noticed. Translators usually do a good job of making the transition feel more natural, but you can spot a translated text a mile away if you pay attention.
To me writing in English is just another part of the challenge. I totally know what you mean when you worry about style and grammar. It's all about the dreaded "I know what this verb/phrase/expression means when I read it, but I wouldn't think of using it when I write". In the end, it boils down to practice and vocabulary. We foreign speakers have a smaller repertoire of words (especially verbs) compared to a native user, so we need more generic words to describe the same concept. Instead of writing the right, specific verb we resort to the same old power words ("go", "get", "be", "take") and slap them next to a few descriptive adverbs. People understand what we mean, but we disrupt the concise flow of literate English.
Slang and accents are our other problem, but I just avoid using them altogether.

One thing I can do for you is point out that if you are worrying about this, you have come a really long way as a learner. When you train with tigers, you tend not to notice your own improvement!
I think there are greater obstacles to writing a novel than the second language. There are plenty of differences between a post and a novel in terms of style and exposition. Posts are often built around character thoughts and feelings, but putting so much insight into a single book passage would bog it down a lot, just to name one. The way the narrator works also needs adjustment... I find it very fascinating, actually!