Found out about W affair, she does not know I know

[Steady9]

To put in perspective we were having sex three times a week for the past year. I never turn down sex. And I would have said we have a great relationship. I think she is a WW... Now out of the blue so to speak I have changed. Since she does not know I know..... I assume I have the let her know so we can face the truth. I assume she will not volunteer the info.