I found that learning a foreign language helped me learn english grammar! And teaching english certainly helps, because I have to understand what I'm explaining. You'll probably find your girlfriend isn't great at explaining why the grammar is as it is, most native speakers don't actually know. I'd suggest getting a grammar guide, unless your book is very clear, because grammar is pretty fundamental to fluency. Good luck, I loved Italian when I briefly studied it.
Much as I love my job(s), I am rather annoyed tonight. I agreed last week that I would proof-read two essays this week for £30 each (very easy money, I'm a good proof-reader and pretty fast when I concentrate). I thought that one was due this week, and the other due next week.
I returned the first one this evening, and discovered that the second one is also due tomorrow. She hasn't finished writing it yet. It's 9pm. I get up at 6.30am, so I need to be in bed by 11 at the latest. I was meant to be going to a party. I am not impressed. I would say I can't do it, but £30 is £30 and I will need food money next year.