How can i get over that my girlfriend used to sleep around?

I'm 20 and i've been with my girlfriend for 6 months now and i absolutely adore her. We're completely in love and both know we want to spend the rest of our lives together. The only problem is that i just can't get over her past. She's from a completely broken family and has been through a lot but what bothers me is the fact she slept with 12 people before me which to me seems like a lot for a 21 year old. Before i met her I'd never had a girlfriend, i was a virgin and I'd only ever kissed one other girl. I can't help but keep thinking about her doing what she does to me to other guys and it sickens me. She says she never had friends when she was a teen so every time she mentions a guy i assume she had sex with him and we argue. I really wanna get over this cos its ruining out relationship. What can i do?