Is it true, what I'm feeling?

Is it true that the dirty mistresses of the world never win? I'm ashamed to admit it but I've been messing around with a guy who's taken. I know, call me a hussie, whatever but please take a moment to see things from my side of the situation. What happens when the dirty mistress starts to actually develop feelings for the taken man? Should I break it off? I know every women deserves to be loved exclusively but I've yet to run across a decent, single man. I'm starting to feel a bit like Scarlet Johannson's character in the movie "He's just not that into you". Help!?