One Slip-Up in Law School

I've been dating Lisa since we met freshman year. We're both first-year law students now and she's the one I want to be with forever, but a week ago, I cheated on her. It was nothing serious, just one night with someone else who came on pretty strong. But it feels like a betrayal every time I think about it. I know this doesn't change how much I care for Lisa, and she means everything to me, but the guilt is overwhelming.