Re: Need some help...
Firstly, it sounds like you need ALL the details of what went on - where, when, what was said, what was done, positions, protection, oral, anal, facials etc etc (sorry for the brutal vulgarity). You need to know how many times during that "one" encounter and who initiated each time. You need to say if anything was said about you or if she even thought of you. I am not sure you are going to get all this truthfully. I am not even sure that she is being truthful about it being just one time. Why would it be ? You confronted her and she admitted what you already knew - standard cheater textbook stuff.
If you found that she had done it at least twice then she would have admitted to just two times. And if you found out about more times after that, she would say she lied to protect you from the pain.
She needs to understand that you don't need "protection" from the pain from her - you need absolute honesty. A polygraph might help. You need to get the truth and the whole truth out of her.
This is my quest, to follow that star
No matter how hopeless, no matter how far
To fight for the right, without question or pause
To be willing to march into Hell, for a Heavenly cause