I need advice..

I’m gonna use this as a journal and am very open to what you all have to say, if anything. Advice, if you think I’m over reacting, under reacting. Please don’t hesitate to put me in my place, cause I need it. Just don’t be mean please lol

My current fiancé and I have been dating since 13 years old. When we were 16, he skipped school with his ex and she gave him head. I had no clue about It, I was out of state for 6 months when it happened and he was the one to call me and tell me what happened. After I came back home from being out of state, we started talking again and working on our relationship. We were basically on and off the rest of high school. Then as adults, we decided to get serious and all the petty problems we had in high school, we got rid of. Weve been going on 3 years strong now. Engaged, we have a dog, and we have a house. My fiancé’s cousin, who happens to be REALLY close with my fiancé just announced he’s in a relationship AND having a baby. And turns out, it’s my fiancé’s ex from high school who he cheated on me with!!! I have not seen her or anything since all that happened. Yes it was 4 years ago. Yes I’m still hurt by it. I still have dreams about it. I have some deep rooted issues. I check her social media all the time, look at her pictures. I hate myself for it. Anyways, now she is having a baby, she is apart of the family. We were just told there’s a Father’s Day party tomorrow and they’re most likely going to be there and I haven’t been able to sleep or eat since I heard that. I’m so nervous to be around her. I don’t want my man around her but he’s so close with his cousin, who is dating her. Every family event we always hang out with his cousin. Idk why but it almost is making me ask my fiancé if we can celebrate Father’s Day a different day without everyone else.. but I don’t want to do that to him. I have anger, sadness, hurt. I can’t stay calm. Is it crazy I’m still thinking about something from 4 years ago? They didn’t even have sex and I’m acting like they did and idk how to get over it. How do you actually move on from something like that? Is it crazy I’m having these feelings? What are some grounding techniques I can use when we go tomorrow so I can enjoy my time and not be a nervous wreck. Please someone 😭😭