Why do I feel like this?!

So my boyfriend and I have been together for a couple years. We had broken up because of a fight about a girl(I was his first real relationship and due to stress at work and the argument we ended it). We got back together and now have been together for a couple years.

He tells me one of his coworkers has been promoted and the coworkers were all going out for dinner at a fancy place. Fast forward to today, I find out through it that this girl we fought about not just there but was sitting next to my BF...

I’m upset about it. He knows how I feel about this chick, but what do I do? I know it may not have been his choice but it also wasn’t a formal work event so wtf ? What do I do? I feel like sh*t and I legit cried about this. Help..