You've already done it.
You confessed and, I assume, apologized. There's nothing left to do but wait and see if he gets over it. He might, he might not, but that's out of your hands. You've done your part. Don't keep apologizing or groveling. This guy isn't even your boyfriend, and it's not like you blew the other dude. True, I wouldn't like it if my girlfriend kissed another guy on New Year's Eve, but then if she were my girlfriend, I would have spent New Year's Eve with her.
Thanks for the q.
"but then if she were my girlfriend, I would have spent New Year's Eve with her"
Yes, exactly. Where was he on New Year's Eve?