<human>: Give me the gist of the text in a nutshell

*This is probably going to get downvoted, but I just need some other points of view on the situation. Not our ages.*

A few months ago, I started a new birth control. It changed me mentally until my body adjusted. I was constantly emotional, depressed, and finding reasons to doubt our (at the time) almost 4 year relationship. I'm not trying to make excuses, just stating my mindset at the time. My curiosity got the best of me, and I went into his facebook and read through some of the messages with a girl he told me he'd **only** taken out on a few dates a few months prior, before we agreed it shouldn't be an open relationship anymore. I feel horrible about ever doing it, and regret it deeply. But he doesn't know.

He knows I like to know his past with other girls. I get curious. 

Tonight on Skype, he got a text on his phone. I asked who it was, just to keep conversation going. It was a random number. After hearing a few more notifications, he tells me its the girl from a few months ago. Whatever. Then I asked if they ever did anything besides the couple of dates. He said that I'm thinking of a different girl. (Same name as the one in the messages, obviously talking about sexual things they had done earlier).

He told me that he met her at some party, and he didn't really know her. Just accepted her friend request to not be a dick.

I know we were talking about the same girl (very distinct facial feature), but he told me that they'd never even been on a date, and that it was a different girl, same unique name. And that they'd never done anything sexual. When their messages clearly say otherwise.

Do I let it go? Do I bring it up and tell him I invaded his privacy? What should I do reddit?
<bot>: Boyfriend's facebook messages say otherwise then what he's telling me. I feel bad for invading his privacy, he doesn't know. Do I come clean and bring it up (he lied to my face), or let it go?