My Law School Experiment

So there was this girl, let’s call her Jess, and she's way out of my league. But one day, I convinced her to come over while my girlfriend was away for work and, well, it just happened. It wasn't anything too wild, some kissing, a little touching, but now Jess has started texting me more, even after I told her it was just for that night. Am I the asshole here? How do I tell Jess it's really over?