What I do ia to keep notes of every sentences I think I might use and try to use it whenever I can as muc as possible. For instance, in italki I read many discussions and I pay special attention to those written in English (my target language) and whenever I see a sentence / structure I like I write it down and use when it is possible. Anyway, understanding is the first step to produce (talk and write) so don't give it up!

I think depends of what you read, is not the same read a newspaper, a magazine, a novel or poetry because the "language" used are different for each case. The next links maybe will help you to get it. http://figurasliterarias.org/ https://es.wikipedia.org/wiki/Anexo:Tipos_de_texto As recommendation try writing basic sentences first and then go forward to other more complex like in the texts.

I have the same problem in English, I think that when you`re reading you can understand the general idea, but when you write, you must do it with correct grammar...

Spanish writing is harder than speaking or listening. There are many differences between english or spanish that can make your speaking really diffucult, like the use of "the", that in spanish can be used as "el (male) / la (female)" for example, or verb conjugation, that is really difficult to dominate. Sorry for my english, I hope you have understood what I mean ^^"