Getting over my girls past sexual encounter

I was in a very loving relationship with a great girl for about a year and a half but then I decided to break up with her after realizing that I didn't love her in the same way she loved me. Immediately after I did this I realized that I really did love her 100% and after 2 months of talking to her I told her that I wanted to have a relationship again and she was a bit reserved at first but we have been getting along great since. I think I will marry this girl one day but I can't get over the fact that when we separated she had sex with another man. This hurt me because it happened so shortly after we broke up and we also lost our virginities to one and other so I saw her as just mine. I really liked the fact that I was the only one to ever experience sex with her but now I feel really depressed when I think about this incident. It even came to the point where I worked myself up into a rage and thought of having sex with another girl just to get over it.