Having found out through the grape vine that something is a miss and my partner is cheating on me or at least was. In fairness I knew something was up and wondered this (that realization that it can't happen to me, has passed, as has the anger). This is a long relationship over 10 years.
I have found evidence to back this up. After asking very specific questions that I know the answers to and the person lies straight to your face, do you put this down to a bad judgement or just lieing to cover their back?
The evidence I have is very good and damming, do I show what I have?
I love this person a lot but also I will not allow to be lied to, for me this is the ultimate sin.