What to do! Help!

I'm 17 and he's 18. A couple of weekends ago he went out to a party and hooked up with another girl and we aren't officially together or anything but we basically are. Anyways I got told by my friend that it happened and I confronted him about it but he kept Lying saying it didn't happen and slowly I would get it out of him and then a couple of days later I said it in a way that if it wasn't true he would have said it wasn't but he agreed that it did happen. Anyways I just can't seem to get over the fact that he lied to me and I don't know how to move on from that, can someone please give some advice as to how I can move on from it.