Intrusive negative thoughts and feelings years later

Hey, This is my first time posting here. I'm after a rough year. I've been in a relationship with my girlfriend for 8 years, since I was 16, now 25. During my first year of college when I was 19 I went out a lot with classmates and friends. I ended up kissing another girl for no longer than 2 seconds. I told my girlfriend a few months later because I felt so guilty. I used cry driving to college some mornings. Anyway she forgave me instantly, saying she had done things she regrets (texting a guy at the very beginning of our relationship). After telling her, in 2015, our relationship has had its best years ever. We traveled Asia, made new friends and get on so well. It's been 6 years since the incident and last summer we bought a house together. All of a sudden it triggered these intrusive thoughts and feelings of immense guilt. I spent months in the later half of 2019 in a bad state, whenever I had spare time all I could think about was how bad I am and how I made such a bad decision. I feel I've grown a lot in the last 2 years as a person, graduating college, getting a full time job and gaining great experience traveling with her. Now I look back at that college period and hate myself so much. One morning I cried in the car park before going to work. The thoughts drain me emotionally and physically whenever they come back. They can go for days or weeks but then suddenly get triggered and haunt me for a while. I can't stop thinking about the event and keep replaying it in my mind. She talks a lot about marriage in the next few years but I feel so guilty whenever that conversation comes up. It's hard and when the thoughts are strong everything else feels so irrelevant. As I type this I'm feeling a little relieved somehow, it's making it harder to describe the feelings. I hope I've got my situation across clearly so that someone might be able to shed some positive light on it or maybe I deserve this. I look at my grandparents a lot more since these thoughts have started invading me and feel like I'm a real fraud. I like to look after them but they don't know what I've done and I feel I'm hiding so much, if only they knew. I don't know is this ever going to go away.

I think that my best advise for you is that we all have made some bad decisions down the road. It. Is okay to acknowledge those feelings and accept them but eventually you have to move on and learn from it. Since, if you can’t alllow yourself to move forward from it it’s only going to stick with you longer and longer. It okay to think about it ever so often but eventually you just will make sure. You don’t do that again. All we can do is learn from the mistake we can’t take it back. It also makes us a stronger and better person than we were before. I hope this helps!! :)