I'm a virgin but my girlfriend isn't.

(Male 21) I've gone out with this girl (20) for 2 months now and she happens to be my first girlfriend ever. She is great! Great personality, kind heart and likes me for who I am. I've never had trouble with talking to women because I was raised with 3 older sisters who encouraged me to not be shy. When it comes to sex my parents have always told me to wait to fall in love for it will mean something greater than a casual hookup. I never liked relationships due to all the drama my friends used to go through in the past. My mom asked me to try getting into a relationship so that I wouldn't be a lonely 30-year-old guy in the future. Luckily for me, I found someone who I feel I can create a future with but one thing seems to always bother me. Her first serious relationship was with a guy who she knew constantly cheated on her. She had sex with him to keep him focused on her and the relationship instead of the other women. I know she was a young 16 at the time but I never gave up my V card when I had women wanting to bone. I know it's a long time but does she deserve what I've held on to for so long when she gave into temptations. You guys will probably view me as an ass whole but these images keep popping up. Out of the 3.52 billion girls in this world she will have me, knowing that no other girl has been with me. But out of the other 3 billion guys in the world there is one other guy that has had sex with her. I can see a random confrontation with her ex and him shaking my hand and telling me in my ear that he has fucked her and put his dick in her mouth. To think my kids will be coming out of a place were another mans penis has entered. I know to most casual sex is normal sex but not everyone has the same views. Imagine if you were in my shoes. She regrets doing it with him but you can't change the past. How should I cope with this to keep our relationship. That was the last time she had sex btw.