What do I say to my her?

I'm in a long distance relationship with someone and we've been together for 2.5 years. I can only contact her by phone because her parents don't like me, so I'm not allowed to see her. Anyway, about a week ago I was hanging with a group of friends, and we were bored so we decided to play truth or dare. At one point in the game, I was dared to kiss one of the other girls. I really don't know why, but I did it. I didn't really enjoy it or anything, but I feel really terrible. And since then, I've been trying to tell my girlfriend, but I can't bring myself to tell her. It's destroying me inside, but I can't bring myself to tell her. Please, help me.