My parents immigrated to Germany in the 70's and I was born and raised in Germany. Although I attended German schools like any other German child and even studied in Germany, I do not feel confident with my German language skills and also make mistakes when I use the language. Unlike in typical German families, there was little to no German spoken in my home during my childhood. So I wasn't extensively exposed to the German language as a child and only began learning the language in kindergarten when I was 4 years old. I would anyway say that I'm a native German speaker. However, I do not speak it as well as my German colleagues at work, for example. For example, I don't have a comparably large vocabulary and I also don't know as many idioms.
How can someone like me improve his language?