I feel like a bad person

So I had a thing with this guy in my friend group for about half a year. a couple months ago we broke it off because he didn't think we'd see each other over the summer and he wanted to hu with other girls. now two months later and I'm with his best friend but we're all still friends and he doesn't seem to care. I thought I was over it but last night I watched him hu with my best friend and I started to feel so sick and basically had an anxiety attack in front of everyone. I guess I'm a good actress cuz I made it seem like I was just scared of the horror movie we were watching. Still, I couldn't stop shaking even when I got home I couldn't sleep. I should be over it. I shouldn't care bcuz technically I did it first. I think the fact that I was there having to watch it happen is what got me unsettled. I know he's hu with other girls this summer and it doesn't bother me and I even gave my best friend permission (there's a story behind that). I feel like I should tell her that I was uncomfortable and probably tell my current guy that I felt that way. I like him a lot and I don't want this to hurt his feelings. I just need advice on how to deal with this and how to comfort myself when I feel like my anxiety is taking over my life.