Welcome, Bella12, and I'm sorry you need to be here but I'm glad you are here. You'll get a lot of support and knowledge from this site. (Don't necessarily judge it from the responses you get on a long holiday weekend, I'd imagine it'll be kind of slow.)
Gently, I think your BF is feeding you a bunch of bullsh*t.
he admitted to the affair saying that he had believed I had moved on away from the relationship and didn't know that I still wanted to be with him. We has extremely bad communication issues in our relationship on my part so I do accept my responsibility. I asked him if he loved her and he said no, that it was all just a game to him and that he wanted to work on our relationship.
This is a bunch of blameshifting and damage control. Of course I could be wrong. However, the more you're on here, the more you'll see there's almost a script circulating out there amongst those who cheat. I know when I first posted here, someone responded saying that there was likely more to the story, and honestly I was a bit defensive/offended, thinking "I know him, YOU don't!!!" However, if I could remember who that responder was, I'd thank her.
My initial post was about a myspace page with one friend, a female, who lived near where my FWH was doing a temporary work gig. Read my profile if you'd like to know how it evolved from there....or should I say, "mushroomed".
Read, read, read, read here on SI. Read in the Healing Library (look in the upper LH quadrant of your screen, it's a link in the yellow rectangle), read in this forum and the General forum. The links given by mysticpenguin are a great place to begin.
A warning, though: at this point I would be shocked if your BF doesn't go underground with his behavior. He now knows how you learned of your information, and he will therefore be likely to become stealthier. It doesn't mean he's stopped. Earnest apologies are not the same as change. You are NOT crazy; one thing you'll learn in a hurry is to trust your gut.
It may or may not be relevant, but I do find myself wondering why you've been together 16 years, have a child, and aren't married? Was that coming from you or from him or from both?
Hang in there, you're not alone with this now.