Most Helpful Guy
There are reasons for why someone wouldn't confess. Without knowing her it is hard to say for sure why she won't admit it. Maybe because she is afraid of you knowing the truth. Right now you only "think" you know she cheated. If she confessed then you would "know" she cheated. She might not be sure how you would react if you knew. Also it gives her leverage of complaining about your cheating, while pretending to be a better person. Even if you can forgive her for cheating she has an excuse to not trust you in the future, while you don't have a reason not to trust her. This makes future affairs easier for her. She can always throw your insecurities in your face. Some people just don't have the moral fiber to admit they have ever made a mistake, no matter what evidence has been presented.