Cheating

Yesterday, I just found out something that broke me. As it turns out, my boyfriend cheated on me the day before, with a girl I know relatively well.
I won't say her name, but I'll just call her Jane in here. And his name will be John.

So, Jane and John made out on Thursday. Apparently they were both drunk, but I wasn't there so I wouldn't know about it. Now, I know it might not be a big deal for some of you but we're teenagers, and right now kissing is all we do. Or, all that me and my friends do anyway.
This was a big deal to me. I actually really liked this guy, and I thought that he really liked me. When I was with him I felt happy, and when we kissed I felt special, like I was the only one.
Well, I guess I'm not anymore.
And I was the last to know. You see, Jane called me up, and told me the news. She apologized, and I said thanks for telling me, and it was okay. You know that moment when you say it's okay and you act normal but inside you feel like you're dying inside and you just feel this empty void where your heart just shattered into dust. Well that's what happened to me. I just hung up the phone, and broke down. I started crying, and I just wanted to throw something. So then I call my best friend, and he was with some friends. So I told him what happened, and he goes,
"Oh, so you know about the John thing huh?" And then I hear his friends in the background going, "Oh shit, she knows about the John thing," It was like I was some little child while they were all adults and they were talking about something that I 'wasn't aloud to know yet'. It sucked.
I was sort of with him since around April, but it wasn't official until September. But I don't know what I'm supposed to do. Am I supposed to break up with him? Am I supposed to just accept that it was a mistake and that it won't happen again?
What am I supposed to do?