I mean the Germanball above isn't really wrong. Language is a skill, and so is writing. Like any skill, one needs to practice it to become better.
Read and write more in whatever language you feel you lack. I'm a monolingual anglophone (I can understand bits of other languages, mostly Spanish), so I obviously consume lots of writings in English and use what I read to help me write.
I see what you mean though, my time spent becoming exceedingly proficient in English means that I have a better grasp of the language than a non-native speaker-- or even other anglophones who really don't immerse themselves in the language beyond simply communicating. That doesn't mean that you are necessarily lacking, though. Just practice.