Hiya! Firstly its completely understandable that you feel like you do! I would be exactly the same if it was my boyfriend telling me this. But what you have to remember is the past is the past. She like you said is in love with YOU not that guy, you're her everything, remember that. I and many others i'm sure have made mistakes in the past and would hate for them to haunt them and ruin their futures, the same with your girlfriend she was a different person then. I know its very, very hard to just forget about this especially as you see them together when you're with them both but every time you get these images in your head just say to yourself 'no, shes with me, not him, we're talking about marriage, shes in love with ME, i have nothing to worry about, everything is ok' and eventually the images will go and everything will be fine! I hope your both really happy together! Good luck x