<human>: Can you provide a brief rundown of the text in within 3 sentences?

I figure it's important that I state that this girl and I aren't officially dating, we haven't defined anything, but I know she'd be hurt if she read this. 

Last night, one week away from seeing my gf again, I had sex with another girl on impulse. It didn't mean anything to me, just something fun to do to pass the time, like video games, but I do realize it was wrong. 

My question is, what's the best way to handle this? Can I put it behind me and simply ask to define the relationship immediately when I get back or what? Any help would be great.
<bot>: cheated" on a girl I really like with a girl I just thought was attractive, and I'm not sure why I did it, or where to go from here