*noddles* I think what you really need to do is get practice... immerse yourself. Studying books etc. can be helpful, but in the end, just learning grammar rules by heart will only get you so far.

Do you get any German TV stations, for example? If yes, I'd suggest trying to watch German news (the Tagesschau, for example); or you could also read German books, or subscribe to a German magazine or newspaper or check if your local library has any (I'd recommend the Spiegel and the Zeit, respectively, although they may not be the easiest in terms of language).

That's pretty much what I did to improve my English after dropping my English classes in school, too. :)