Fiancée slept with someone before we were exclusive

I’m currently engaged to a fantastic girl. We have been together now for 3 1/2 years total. We recently got engaged this past December. We started talking and first met back in August of 2017. We hung out a few times and had sex but we were at different colleges. Over the next few months we continued to talk and I visited her once or twice (it was a 7 hour drive). We were getting very close and even calling each other babe and such. We started dating in the middle of December that year. I had asked her to date before that but she had said she wasn’t ready yet. A few months later I found out that she had slept with someone a few weeks before we started dating (beginning of December). She had slept with this person drunkenly and nothing happened around thanksgiving, but then she did it again a week later and they hooked up. When I found out, I told myself that we were not together at the time so I would be able to get over it. It has come up here and there over the last few years and every time it makes me anxious and somewhat angry. I just can’t understand why she did it, she was drunk both times and had a high school crush on the guy but I still thought we were more than what we were at that time. She has apologized numerous times and told me it was a mistake and she would take it back if she could. She has been extremely loyal since then and we are very committed to each other. If I could erase it from my mind I would because I feel that I can trust her fully now. I was just wondering if anyone had any tactics or anything to deal with this? Or is it just time that will heal it? Some things just trigger these feelings (show with someone sleeping with multiple people, etc.) and give me anxiety..