English lessons and the temptation to rewrite a book

As some might have notice English is my second language. I have managed a certain degree of proficiency reading and speaking it, though not so much writing, which seems to be the highest peak to climb. I still take English lessons at work to improve but it seems that at this stage improvement comes really slow, so slow that you barely notice it, that is if you notice it at all. Somedays it seems to me that I am going backwards as I make stupid mistakes about things that I have known and learnt ages ago.
Recently I was hired to translate a book from English to Spanish. It's wasn't a terribly difficult job though I met a few problems: first it was about a subject I wasn't totally familiar with, business administration, something the Spanish civil service is allergic to. Secondly, the book wasn't very well written, actually some parts of the book, as it was a collection of documents from different authors and therefore every chapter was written in a different style, and so some were better than others. Also there were a few expressions that were new to me. Here I am thankful for my wonderful literate husband.
The most difficult thing to achieve was to put into Spanish something that made perfect sense to me in English. Here I pondered about what a perfect translation should be. Should I be as faithful as possible to the original? or should I rewrite to make more sense in Spanish but trying to keep the spirit of the phrase but without any resemblance to the original? The temptation to rewrite is strong, specially when the original is obscure or poorly written. Could I say better? Am I being too proud? too critical? Am I supposed to be critical at all? Could I make it actually better?
I tried with my best intentions to remain faithful to the text, to correct what I thought will improve with correction and to preserve the spirit. To which extend I succeded I don't know yet. I have not received so far any comments on it, which I cannot tell if it is good news or bad news, but for the sake of self-improvement I would like to hear some comments on my work.
In the mean time, I should keep on working on my English and maybe one day I might become as proficient as Joseph Conrad...(in my dreams).