Welcome to SI, stillcantforgive; I'm glad you found us and hopefully you'll be able to get information and help here that will ultimately lead you to peace. 
I can understand the anniversary (antiversaries as they're known around here) bringing it all churning to the forefront again; not a good feeling.
Gently, one thing a lot of people who come here quickly learn is to trust your gut. I think that's a little harder for those of us who have a natural bent to worry a bit more than we think others do. However, don't let that keep you from listening to what your gut may be telling you.
Sometimes people's stories resonate particularly with me, and elements of yours does. When my fwH and I met, we were together every day thereafter. We had the cheating conversation; I came with the shadows of prior relationship in college where I learned my boyfriend from many states away had a hometown honey the whole time we were together. As an aside, I was pretty naive; I thought if someone did all kinds of things to get you to date them, they must be available --not involved with someone else. I'd lived a sheltered life up 'til that point, I guess...small town, small high school, word gets around and you know everyone else's mother, so there was more accountability or else things must've have been really far underground.
My fwH found time to cheat on me while we were dating, too; I kind of caught him, and he apologized and gave me the 'I don't know why I did that's and so forth...and though hurt and scared, I thought we worked through it, that cheating had been stomped dead like a bug on the sidewalk. What I didn't know then is that generally speaking, cheating is like a dandelion...it has deep roots.
Why did he kiss and grope another woman? Why did he lie about having to get up extra early the morning after your conversation about cheating (was that on your b'day or a different day...wasn't quite clear). That shows intentionality to me. What had his plans been for that night?...there must've been plans if he was unable to stay?...or am I misinterpreting the timeline?
You say, he "told me it was a mistake and never ment for it to happen, and that he didnt know why he done it". Well, he didn't invent that line for sure. However, in my opinion, the barest minimum for moving beyond a betrayal begins with the betrayer figuring out the 'whys'. You don't want to think you've moved beyond something only to discover the only thing he's figured out is to not forget to log out of Facebook...or email...or whatever. Believe me on that one.
When I came here, it was as a result of looking up his email address on mySpace...like you, I can't tell you why. You're welcome to read my profile (you click on the smiley face) for the full Russian novel version. That's ALL. He had a mySpace account, and one friend, a female.
Suffering a betrayal turns your world on its head; your reaction and concern is entirely normal. You're not married to this guy, and I don't know how long you'd been together when this incident occurred, what your exclusivity was understood to be (I read that you definitely believed yourself to be in an exclusive, monogamous relationship), etc...so absent of all that I'll zero in on the LYING. Do you want to be with someone who lies? What has changed for him that he won't lie again?
I hope I'm not terribly negative; forgive me if it seems so. I remember when I first posted, I got a response that was pretty blunt and I thought that person must just be projecting their experience onto mine. Certainly possible. However, in hindsight, that was the most beneficial thing much as it rubbed me wrong at the time.
I'm sorry you're struggling with this.
