Obsessing over girlfriend betraying me

Okay I’m reading this story on Reddit and looking through the comments and now I’m nervous. The story was that this girl dated this guy for 3 years and really liked him. But they broke up because they went to different colleges. She then met a different guy and was dating him. But then she gets invited to a wedding as the made of honor and this ex (who she had not seen in 3 years) is gonna be there as the best man. They have to walk down the aisle together, have a first dance, and then take photos. If I was in that situation with my girlfriend and I watched her dance with her ex (the only part that bothers me) at a wedding I would feel very uncomfortable. I read some of the comments and a lot of people are saying that makes me insecure. I have ROCD and I can definitely obsess about future situations where I feel betrayed by my girlfriend. I would like to emphasize other then the ocd I’m pretty secure, don’t believe she cheating on me, don’t control her, and we’re are doing very well. So I’m not looking for advice on my relationship. my fear right now is that if I brought this to my girlfriend she would say I’m insecure and controlling and hate me. That this exact situation will happen to us and she will pick dancing with the ex over my feelings and it will hurt really bad. I’m also feeling like it’s not ocd and I’m just a horrible and insecure person. I can’t stop thinking about these scenarios. Typically it just spirals into worse scenarios in my head and I have to try really hard not to go to my girlfriend for reassurance. Because going to her for reassurance usually just makes things worse and puts strain on our relationship. Especially since I’m pretty sure I’m being irrational right now, but it feels very rational and very real and I just need her to tell me in perfect words that it would never happen. But if she does do that I just move on to the next worse scenario and I don’t feel better and it never ends. I didn’t ask her for reassurance the last couple times but it’s hard, I feel like I need to prepare for when it does inevitably happen and she betrays me. I guess I’m just wondering if anyone relates and has a similar story? Also is feeling some type of way about the ex real or ocd? Also any tips to not tell my girlfriend this because I realllllly want to but I know there is not point in bothering her about hypotheticals.