If you can remember SVO and V2 I think you can nail the grammar, after that it's remembering plural forms, genitives, etc, and vocab ^^
I'm pretty much Arabic and know Arabic, obviously, English obviously x2, some French and most of Spanish. I spent around 2-4 months and I'm getting the hang of things. I just need to work on a little of my grammar, and to know how to say all the numbers in Spanish ^^, although I may have some Spanish wrong without me knowing :P.
I kind of feel proud of myself somehow
I have a bit of experience in this area. I moved to Sweden at 4 years old with my family. We lived there for 5 years and I went to school in that time and was fully immersed in the culture and everything.
Parents are Aussie so we spoke English at home but out and about and in school we spoke Swedish. I moved away in 2005 and have been in Australia for 9 years straight now. So in my case I learned by immersion and integration but that's not really the question here.
I'm in Sweden atm visiting and although I've developed an accent (whereas when we lived here I spoke like a native) I have excellent Swedish still and lots of people complement it and ask how it's so good after so long.
This is my third visit since we left and I don't really make an effort to work on the language when I'm in Australia but when I do I've found reading the newspaper and watching Swedish TV and movies is the next best thing to immersion.
Good luck with it and don't give up. Shoot me a message if you want to talk more.