You are fighting a lost cause. Your girlfriend is one of those many unfaithful believers that want to still think of themselves as JWs but are driven into a hidden life for the sake of pleasure. You do know that she can be excommunicated for simply being with you in the same apartment overnight? That is enforced by elders who, when they suspect something is wrong (Read fornication), keep the residence under surveillance, from their own vehicles, overnight.
Once she is excommunicated she will be shunned by everyone she knows in the religion including family members.
She will keep her relationship with you so long as she's out of the Witnesses but she will eventually dump you once she has been accepted by them. In many cases the penitent JW will not even give the time of day to their former lover. This situation has been played out countless thousands of times in JWdom.
Prepare in your heart to live without her. Her actions show that she will not respect you as a non-JW.