My suggestion is a really dangerous and very effective. Please take a careful consideration in this.
You might want to talk with your two or three friends with a van to have a mock break-in your house while your GF is in the house. That will surely give your GF a reality call. You'll might have to deal with cops responding to this call. You'll have to pay some of $$ for few of broken windows.
I don't really think your GF will want to stay with you after all of this, lol. At least I gave you something to think about.