Just what do you expect to hear from the folks on this forum board by confessing your poor sense of integrity and your lack of personal boundaries.
We're not priests. I suggest you confess to a priest or get the ballzz to confess to the person you're betraying and let the chips fall where they may. Hey, if she breaks up with you then you're free to get jiggy with the cow the plays with you knowing you're in a relationship... she DOES know you're in a relationship, right? Tell us you're not that much of a douche-nozzle by lying to her too.
“The willingness to accept responsibility for one’s own life is the source from which self-respect springs.” ~Joan Didion