Get yourself some good legal advice and like others have said, try and lock down your assets.
Same thing happened to a friend of mine (except his wife was cheating on him with her ex), same thing - broke down, confessed everlasting undying love, I'm an idiot, please forgive me it was just a horrible mistake, my life is yours etc etc etc.
My mate loved her, thought their 5 year old was his - he was torn up but he gave her another chance.
8 months later another friend give him the heads up that his wifes car was parked outside her ex's house all day. Another confontation, more tears, more confessions more forgiveness - seriously - my friend wanted the fairy tale and was willing to give anything for it.
Fast forward another 5 months, same thing but this time finally he says enough is enough. The little boy is now 6, has had his first day at school, it was just a horrible situation. The wife saw him as a provider sure, but nothing else. I don't know what the hell is wrong with some people. He's now a broken man, has started drinking, isn't working anymore and I worry for his future.
Don't let this happen to you bro. Do something now, but your girl, she had her chance. Don't reinforce her behaviour by telling her it's all ok and you'll let it slide. It won't stop.