It's understandable that you would feel guilty. When we do something that goes against our moral compass, especially if we know we should stop but continue doing it, it can often leave us with guilt and regret. Unfortunately, you can't go back and undo what happened that night, but you can learn from it moving forward. You know it's a situation you won't want to repeat in the future, and that's a good thing.

Try not to beat yourself up about it too much. You aren't the person who cheated on their girlfriend, and you can't own his actions for him. In the end, all you can do is be responsible for yourself. It's important to be able to practice compassion and forgiveness when we make mistakes, for we aren't perfect, and we will "screw up" from time to time. The key is to learn from it, forgive ourselves, grow, and move forward.