Umm. First of all you have to think very carefully about your feelings for both your bf and the guy. Did you kiss the guy because you really liked him a lot? Or did you kiss him because you were upset about what you "assumed" happened btw you and your bf? (Sorry if that sounded offensive...cos there are people who kiss other guys cos they're upset over breakup's etc. It's just a scenario, nothing personal). Or was it just a little fling that meant nothing?
Then you have to think about why you just rashly assumed you and your bf were over without even asking him. Was it because you were "losing your feelings" or faith in the relationship as well?
Right now, I think your bf would be very confused too, if not more confused than you, because he probably wouldn't understand why you kissed that guy. You have to think carefully about whether or you still love you bf enough to want to keep the relationship. Because unless you're sure you do, it's very unfair for him and ultimately, he's the one who's most hurt.
Take some time alone and think about everything between you, your bf, and the guy. Until then, I don't think it would be much point or fair of you to talk to either of the guys cos you're so confused with your own feelings as well. Meanwhile, you should probably tell your bf that you need some time to sort out yourself, in case he gets even more worried that u're completely ignoring him.
No matter what you do, you have to make up a choice quickly. No one can tell you who to choose, or what to do with the 2 guys, cos no one knows your feelings better than yourself. Someone will get hurt in a situation like this, but if you really care about the 2 guys, you should try and minimise the pain on all three of you.