I speak English as a second language, and don't use a proofreader for Grassblades (though I probably should; they could help me catch spelling-mistakes, if nothing else) - and I don't live in an English-speaking country.
However, at this point, I am confident enough in my language-skills that I don't desperately need a proofreader. I know my spelling, I know my grammar (I can, and have, debated the use of the Oxford comma, for example XD), and I'm well-versed enough in the language that I even feel comfortable with most of its idioms and quite a few of its dialects.
I got to this point by reading. I am a bookworm of truly epic proportions, and as soon as I could, I started reading novels in English - and haven't stopped since. I read a lot of books on all kinds of subjects, ranging from the classics (I'm working my way through Moby Dick for the second time!) to modern scifi. It helps me build up a vocabulary and get confident with the usage of words both in dialogue and in description. It also helps that Swedish TV imports a lot of English-language programming, and doesn't dub it. I grew up reading subtitles, so I got comfortable with hearing spoken English as well.