How can I improve my writing fluency in English?

Reading widely can certain support your writing but it seems you already read quite a lot. However, from your example it seems that your reading predisposes you to more complex and formal sentence structure than is appropriate in all contexts. So including some fiction and informal correspondence in your reading may help. Listening to conversations would also be a good supplement - podcasts are a good source of semi-formal language. Also, remember the level of formality and approach to writing is as much cultural as linguistic. This post by Francois Grosjean illustrates that nicely: http://www.psychologytoday.com/blog/life-bilingual/201408/bilingual-challenge.

Ultimately, the only thing that can improve your writing is practice in writing. There are many guides that will suggest structured programs such as this http://www.amazon.co.uk/Build-Fluency-Muscle-Six-Week-Programs-ebook/dp/B0055WYDQA.

But what it boils down to is: write regularly in small chunks in the genres you want to improve. Write even if you don't have anything to write, just for practice. You can even start by copying texts, then reformulating some of the sentences, then starting with mini essays and finally move on to longer texts.

Following a structured program will help because it will make it easier to measure your progress. But just allocating 15 minutes a day to writing (with gradually increasing complexity) should give you a boost.