A night school for my english would be good. So I have only my school english. years ago....
I didn't do any at school. I joined what was supposed to be a beginners class. They didn't have enough people to run the course, so merged three classes together, with people from beginners right up to one lady who did GCSE and got A*.
The problem I have now is finding a suitable class. I have some basics missing which would make GCSE difficult, but I'm already ahead of 2nd year. For example I can speak past (ge...) and future (werden) tenses which are beyond 2nd year, but then I struggle at times with die, der and das because of my limited early learning.
I know... while I'm sitting here typing this I could be learning instead. But I'm too lazy... 
BTW your English is excellent.