A Chinese friend of mine got a high IELTS score, 7.5, and then went to study for her Masters Degree overseas.
When she came back, her colleagues complained that they couldn't understand her when she wrote in Chinese!
You are right, you need to think in a different way to communicate well in the other language.
But if I can make one suggestion ... when I was a kid, my English teacher used to get very annoyed if I didn't pay attention to proper punctuation and capitalisation. Now I am an English teacher, and as a general rule, the students who try to get the small things right will also get the big things right eventually. If you don't think that the basic rules of English are important enough to follow, how can you ever be a good writer?
So maybe the first step for you would be to review what you have written and just correct all the obvious mistakes. Then practise double-checking as you write, and correcting yourself as you go along. Once you have the basics under control, it's a lot easier to move up.
OK, I'm lying about it getting easier! Writing a good composition in English is very very hard. Most native speakers can't do it, even after studying English as their native language for ten or more years. In a sense, that's why the basics are so important. You need to get the basics right, because it's very hard to get the advanced stuff right. If you have the basics, at least you have something. If you write an OK composition with poor basic grammar and spelling, it will be a terrible composition. Even a great composition with bad basic English will just annoy native-speakers and they will not read your words.
Sorry if this sounds mean, but it's the truth. You need to use a capital letter at the beginning of every sentence, every time, no excuses. Every good writer does it, so if you want to be a good writer then you have to do the things they do.
I found reading helped a lot when learning English. My writing skills improved tenfold as I read more and more.
Hoped this helped,