I think that this is one of those things that are just not do-able in the instant moment. If your girlfriend doesn't know this already I think it is fair play to tell her that what she has asked of you is impossible. Try asking the same of her and see what she comes up with!.
The only thing you can do to show her that you love her is to behave in a consistent and loving manner over a period of time.
I think there is much to be said for the old addage 'Handsome is as handsome does', or to put it another way - Anyone can talk the talk, but walking that talk is a different matter. For instance if you say that fidelity in a relationship is importat to you, then don't screw around, or if you despise domestic violence don't be drawn into giving her the odd 'slap'.
You may not be sure yourself if you do love this girl, but if you do you will find that behaving in a loving way comes pretty easily. if it doesn't then at least it answers the question.