Ex boyfriend and ex friend

Long story short my ex and I were together for 4 years, he was my first love. He cheated and he was emotionally abusive. His cousin is married to my best friend and we all grew up in a small town. So I have to hear about him or possibly see him when I go back and visit the town. He sometimes goes over to my best friends house since he’s cousins with her husband. I feel resentment when I hear about this, especially when he bring one of my ex best friends around and my ex friend has to talk to her. (My ex friend and I weren’t friends for years before this and it didn’t end badly, but she also tried digging for info out of me when I didn’t even know about them)He recently had a baby with my ex friend, and he even screwed her over too. But they are back together now. I am married and have a child of my own. But, I am still so bothered by all of this. When my friends like pictures of them together, I feel absolutely hurt. I feel like they are against me. I know this isn’t the case deep down, but idk why I’m still hurt by all of this. It makes me feel EXTREMELY guilty. Why cantI be happy that they are (currently) happy? I could delete them off social media but I would just go find their page and look anyways once in awhile. It makes me feel sick watching them be happy together and going over to my best friends house. WHY AM I LIKE THIS??? It makes me feel disgusted at myself for even still being so bothered by it. Like he wasn’t even good to me, so I don’t get it!!!