You write English well enough for general purposes (and maybe for ESL clients) but definitely not well enough to represent yourself as a professional writer to anybody expecting native speaker-quality English writing. I don't know that it's something you can necessarily improve on.
There's nothing terribly wrong in your writing but your sentence structure, vocabulary, and punctuation are definitely obviously ESL. Considering what you posted is likely something you've been very careful to make sure represents your best work, you should never fail to disclose that you're an ESL writer. It's pretty good for an ESL; it cannot possibly compete with qualified native-langauage writing on any level.
Basically, what you posted would be the starting point for a competent ESL author who still needs a native speaker to improve on his writing for presentation to a native audience.