English is my second language. I don't write in German. I write in English since I find it easier to express my thoughts in English. Thus, I have to think about every sentence I put down. My English will never be perfect. In particular, punctuation is a big problem, I know. But the more I appreciate the readers who read and like my stories. I have not yet found an editor. I guess it's a tedious work for a native speaker of English to edit the stories of a non-native speaker of English.
I agree, though. An editor can help improve the story. The basics - grammar, punctuation etc. - as well as character development, flow of story etc. I think the editor should have an interest in the genre. I write historical fiction. An editor interested mainly in Science Fiction would not be the best choice, I think.
I put aside my stories for quite some time, months sometimes, before I read them again. It helps me to see the typos, mistakes and all the crap I put down.