<human>: Can you give me a shortened version of the text?

I was at a party with friends, my girlfriend left early. I was staying the night at the friends place. During the night, I was lying next to my best friend (she was my girlfriends best friend and is now my best friend as well), lights were off, we both were just next to each other and I don't know how or why it happened, but we both just kissed and then we continued kissing for a bit.
If I tell my girlfriend, she will be heart broken, she will lose her best friend and her boyfriend at the same time. Her past relationship ended because the guy was cheating on her, but that was serious cheating. So I really don't know what to do. I know I can not tell her right now, because her finals are going on, and she is super tensed for them, so I can not put her through this right now. But if I don't tell her right now, it will be very late. 
She is an emotional person. I know this will break her heart if I tell her. But if I don't it will keep on bothering me. And I know she will most probably stop talking to the both of us and will hate us. I really love her, I do and I have no idea why this happened, but it did. And now I don't know what to do.
We both are the kind of people who think cheating is the worse thing one can do in a relationship, and now I am one of those people now. 
Please tell me what to do?
<bot>: Kissed my girlfriends and my best friend (it just happened, never wanted to cheat on her), should I come clean or should I keep it from her?