I was in a similar situation with my wife.First things first.
Does she love you? I don't mean that she says she loves you, but rather do you feel loved (come on, you can overcome denial and listen to your inner voice because for the most part you can see through the words and listen to the heart).
Second thing; women lie, i can't be any more direct on this assertion, they lie and they think(as do we when the shoe is on the other foot and we are the ones cheating).But affairs are a messy thing and though she might think that she can have her cake and eat it too, affairs always leave a rather impossible to miss trail after you overcome the denial face. In my case i knew about the affair long before i had any corroborating evidence, but i went to great lengths to dismiss it and explain her behavior , to the point where i considered going to a psyquiatrist for i thought i was crazy.But i finally broke through my self deceit.It was not a pleasurable experience to say the least and it tore me up inside as if i had a limb torn off of my body in cold blood.
Still i aproached my wife and in a very tactfull and as calmly as i could told her that in spite of her less than truthfull facade i knew what she was up to.Still she denied it, still i didn't give up on her.
It wasn't until ,through some clever sleuthian work on my part, that i got enough facts about her misdeed that i got her to admit the truth.(There were many phone calls where the other party would hang up the phone when i answered, she also got a beeper wich she cleared every time after it would go off. I resorted to ,without her knowing, cloning the beeper's code and trcing back the numbers that called. I am an electronics proffesional so i guess i got an edge on that subject, but anyway after having the details and the identity of her lover i confronted her again and told her i was ready to leave her she broke down crying and sobbing uncontrolably and said that she was sorry and ashamed for what she had done, that she loved me and woul never see that person again. And she kept her word.
It took a long time for my heart to heal,i was sad and bitter and resentfull for many months, but eventually we rebuilt our relationship and are still together after seven years.
It will never be the same.Sometimes i feel like i am in a second class relatinship.But i try not to dwell on the sadness and forget the past and things have slowlly returned to normal.
Your situation may be similar to mine, or not.But the choice is ultimatelly yours.If you break through to her heart you may find that love is still there, but it will be painfull regardless of if you stay or go. Good luck, there are no easy choices.