I have been learning English for more than 30 years that includes at least 15 years of full-time intensive learning in school. I think I don't have much problem reading and understanding an English passage but what I am really weak at is writing and expressing what I think in English. I almost read English every day on the internet and write here and on other forums but I still feel so pathetic at expressing things in English. Do you have any suggestion on improving my English writing?
Do you consider yourself a good writer in you native tongue? If you are, then the issue is translation in your mind.
A good exercise is to read a passage from a book or newspaper and then summarize it in your own words. Pick a topic that you are interested in, because you will likely already have an adequate vocabulary.
If you are not a good writer, then you need to learn about generating ideas, brainstorming, and organizing your ideas in a logical way.
Some people find success by hand copying well-written articles word-for-word and then memorizing a few phrases from it that catch their eye. You might do this once every week or so. It seems crazy, but for some people it works well.
Read the classics. Read textbooks and encyclopedias. Don't read unedited online junk like Wikipedia and Twitter and blognews. Read, read, read. You say you get plenty of practice in writing, so what else is left but reading? Speaking is an entirely different matter. Take a sit-down college writing course in meatspace, then take another one. Read philosophy and logic to work your mind. People tell me the brain is like a muscle. I always tell them, "Your brain is like a muscle." Your brain is like a brain. Learn all you can about everything, especially grammar, style and composition. Hope this helps.