Rumination and guilt cycles

Hi all I’m suffering very badly for rumination and self blame / guilt over things that happened in the past and I had reconciled.

I’m very, very happily engaged. I flirted with someone else about six months ago (it was harmless, nothing happened, I shut it down). I realized it was because the attention felt good / and I was flattered. I was open with my fiancé about this and the guilt I was feeling and we had a great conversation.

I was able to let it go for a few months, but about a week ago the thought about came back up and it is ravaging me. My brain has convinced me that I’m a horrible person and destined to hurt my partner which is making me feel scared of getting married. I feel like I don’t deserve anything, I can’t stop replaying over and over how I could have shut down the advance more firmly. I can’t seem to stop feeling nauseous and anxious. It has led me to feeling disconnected in my life and in my relationship. I know there’s no point in brining it up again and we all learn from mistakes my brain is just making me hate myself and I can’t let it go.