I've been with my girlfriend for almost 4 years, I'm planning to ask her to marry me soon. My problem stems from a stupid mistake I made almost 3 years ago. At a point, our relationship was really uncertain and we were doing the long distance thing for a few months. I was weak and cheated on her, I obviously still regret my mistake immensely, but I haven't told her. Two months after I cheated, she cheated on me, but she told me right away. I forgave her, seeing it like the universe/karma getting back at me. Since then we've had one rough patch, but I'm 100% positive that I want to spend my life with her. Now the actions of my idiotic past self are coming back to haunt me, and I feel she has a right to know, but I have no idea how to tell her. Advice?