I just read a book someone on here recommended to me, thanks to him, I see speech anxiety differently, it might help you too, I read it for free on amazon kindle its called Speech Anxiety To Public Speaking by Lee Ovett. I assume that with your girlfriend you are so comfortable that you talk so fast and dont think of what you say unlike with strangers. I recommend two things that helped me out, talking slowly, reading out loud, and thinking of what I want to say. Hopefully this helps you!