Overthinking my girlfriends past

I have a wonderful girlfriend who has been a Christian for a while. We are saving sex for our marriage. Before she became a Christian she lived according to the world. I have been a Christian my whole life, and had known that sex before marriage is a bad thing. However I struggled and had oral sex a few times with my ex girlfriend. My girlfriend recently revealed to me that she had 4 sexual relationships before me when she wasn’t a Christian. She regrets the relationships and she was also quite young back then. However the thought of her having these sexual relationships with others keeps coming up in my head. It brings up emotions of anger and jealousy. I know it’s not okay but I don’t know how to handle it. I really want to stop overthinking.