Being a lady myself...I say tell her. If you love her at all, you owe her the truth. Yes, it will hurt, but guess what? You already hurt her by cheating. She deserves to know the truth, know who you really are, and make her own decision on whether or not to stay with you. If you respect her at all, treat her like an adult and be honest with her.
Be honest, don't try to justify your behavior, just let her know that you did betray her, that you are truly regretful, and then wait and see what she says. She'll probably need some time to think. She'll probably be furious, hurt, and upset. Let her say or do whatever she needs to to express her emotions, and if she wants you to leave, do it. You might be able to salvage the relationship, but at that point, it's really up to her. Give her whatever time she needs, whether that's a few hours or a few days.
After that...It's up to her what she wants to do, whether she thinks she can forgive you or ever trust you again. But stop lying to her. Every lie you tell will only make it worse when the truth inevitably comes out.
Also, I'd suggest doing some self-evaluation to figure out why you cheated; was it the situation you were in? The person you were with? Something else going on in your life at the time? Think about why you cheated on her, and whether you're likely to do so again if given the opportunity. If you think you are likely to cheat again, you should probably break up with her, since she deserves to date someone who can be faithful.