Child of Adultery

Looking for some answers. My wife just admitted committing adultery 13 years ago - within a week of our 17th wedding anniversary - and that our (now) 12 yr old daughter is a product of that affair. She SAYS it was a one-time thing with sympathetic co-worker, but I don't know if I should believe that or not. I've learned that most of these "emotional" affairs with co-workers are not one-nighters, but longer relationships. She had always denied the affair - even when I confronted her days after it happened. I had suspicions, but NEVER in my heart believed that she had cheated on me. So, when she vehemently denied it, I decided that she had NOT, and let it go. To make things worse, the co-worker was a slow,weird, strange guy that every one felt sorry for. Not one single person - much less woman - would have anything to do with him. He worked in the church kitchen with my wife. She says he does NOT know that he is the real father. This just makes the whole thing even MORE humilating, knowing what kind of guy she chose to cheat with. My wife of 17 yrs (at the time) cheats on me with the town half-wit that no respectable woman would even hold hands with, much less have sex. It's not enough that my wife is a cheat, she's a STUPID cheat that gets pregnant. And the only reason she told me now, was because she trapped herself when we were arguing. Once it came out, she told me everything - or so she says. I'm still not convinced. She says it was not premeditated and that it just "happened" and that she was "stupid" and it was just a "mistake." And, of course, even though they used a condom (so she says) - she still got pregnant. She says it just happened the ONE time - which I still find hard to believe. To her, it happened a long time ago and is old news that she got over a long time ago. To me, however, it's like it JUST happened - and SHE doesn't understand that. So I've raised this bastard child as my own, never knowing until now. I've paid ALL the bills, loving this child as my own - all the while my wife knew better. My questions after all of this: HOW do I come to grips with this and stay married to this woman? How can I EVER trust her again? How do I NOT let this affect the way I treat the child? It's not her fault. Do I let her (and him) get off scott-free, with no one ever knowing except me? Do I tell others in the family? How could she deceive me like that? The deception hurts worse than the actual adulterous act. - Socrates