Ok, so let me get this sorted out.
You started dating Chick A
At the start of the reltionship with Chick A, you made out with Chick B
One year later, Chick A finds out.
Hmmm. First, I hardly consider kissing someone cheating, but to each their own. It wasn't like you were running around on her the entire relationship, hitting anything with two legs and a wet hole. You made out with some chick a coupla times, and that was it.
Second, if you told her you did it, your an idiot. If one of your friends told her you did it, kick them out of the circle of trust. If she found out by luck, then well, s*** happens. If you explained the situation to her truthfully, and she still dumped your ass, then shes way over reacting. If you denied it, then she produced proof, then your boned. Just own up to it, say you were drunk at a party or whatever, you made out with some chick, and that was it.
Anyways, I subscribe to the Russ Martin school. Its all about bumblebees and slut tulips. If you really love the one your with, its not that you wont cheat. You just wont get caught.