I'm going to break up with my girlfriend for cheating even though I had previously
So you did the same thing as her?
Yes but what happened was just a one night stand, she however had the audacity to have sex with another guy during our relationship!
I think that both of you are in the wrong. Cheating is not okay from either side.
You don't know the true story.
I don't know, but I believe that it's common sense to know that cheating is never a good idea. Do you think it's worth the pain on both sides, especially for her? Perhaps you can go to couple's therapy to sort things out.