So you are not only hiding the fact you did this chick before dating your girl, but then cheated on her with the girl? Either cut your losses with the one who's supposedly "obsessed" and just be with your girl, or face years upon years of more of this bullshit, or your girl saying **** it and leaving.
And if you told him you'd never cheat, don't try to make it seem better with "I've had plenty of opportunity". You did it, and that's all that is going to matter to her. Tell her and move on. Otherwise, she WILL find out. The number of times you resisted doesn't mean shit compared to the single time you didn't.