I have been with my girlfriend for well over 10 years now, we have been engaged for a while but neither of us have ever been in a rush to formalize anything with actual marriage. We have had long and very detailed talks about this so I know she definitely feels this way too.
My home network is configured to log all chat traffic (for my own chats rather than anything else) and over the Christmas break I was reviewing the logs to check on a discussion I had recently and noticed that there were some chats logged between my fiancé and another man that we both know that sounded a little suspicious. Not normally the jealous type I tried not to read anything into it.
Just before the new year she "went out shopping". On her return she showed me what she had "bought" (some clothes, nothing fancy) and left it at that. Later that evening when tidying i noticed the receipt for the item, and it was paid for using an Amex card. I deal with all our finances so checking receipts is not unusual. We only have visa and mastercard in the house so I questioned her about it, and after some denials, she finally admitted that it was a gift from this man. I then pushed her about the fact she had met up with him without telling me, and her exact relationship with him, and this is when the revelation came.
It turns out that 5 years ago, during a period where she wasn't feeling great about herself and following a rather argumentative Christmas she had read an article that suggested that long term relationships had a high probability of failure at the point we had reached at the time. At this point the man mentioned earlier (a colleague from a previous work place who she had kept in touch with, but nothing significant) started talking to her more often and asked if they could meet up for a drink. They met at his place, he made a move, and she claims she was so confused about us, and what was happening that she just went along with it and they ended up having sex. Apparently he took some pictures of the events too. Afterwards she figured that was it. Shed screwed it all up and assumed that i would find out and end everything.
This happened a total of 4 times over the course of a year (basically every few months), and hasn't happened since. She has spoken to him, and met up a couple of times after this happened, and he has tried it on again too, but not pushed it. She claims she has tried to refuse his advances, and end it, but that she didn't want to deny him out-right in case he used the photos as blackmail or something. They have met up a few times since then, only to talk, although he apparently tried it on a bit every time, but didn't push it.
We have talked in detail about what happened, but as it was over 5 years ago now, its difficult to remember everything exactly. I think i understand how it all happened, but she is still struggling with why it happened and more importantly why she let it go as far as sex.
She claims she had some feelings for him at the time, but there is nothing there now. I want to believe that but its hard when she has basically lied to me for 5 years (over a 3rd of our relationship).
What should I do?