I've never had this issue before because I've never been in the situation but an idea would be, don't speak your native language unless you have to or unless you are correcting your girlfriend, and she can do the same. I'm assuming you speak English natively and she speaks Japanese natively. So you only speak Japanese to her and she only speaks English to you. If you say something incorrect in Japanese she can either speak to you in English or Japanese(preferably the latter) and tell you the right way to say it. If she is speaking English and says something incorrect grammatically, then you can tell her the right way to say it in English, or if she doesn't understand, attempt/learn it in Japanese to tell her what was wrong with that sentence. An example would be you ask, ogenki desuka? She replies I am well. Or something along those lines.

A mere suggestion.