Hi escart,
Sorry to hear your predicament.
It sounds like the classic 'Illove you, but I'm not in-love with you' scenario.
She is basically asking your permission to have a fling, and then when the excitement is over return to you. But the risk is that it will happen again if you tolerate this.
You really need to point out to her what she is going to lose, as she is depending on your love for her to still stand despite her appalling behaviour.
I'm sorry but I agree with everyone else here. Tell her it's over, that you deserve better and that you also need a chance to find someone that will 'light your fire'
Good luck honey.
Missy
xx