Work with ex, have to hang out with new bf

GF told me she didn't want to be in a relationship anymore because she was still processing her divorce. I was like "fair enough". Couple weeks pass by, I find out she's dating someone else. Tells me "I didn't see this coming". Ugh.

We work together. And she won't stop talking to me. She is always chatting me up at work and outside of it. I am pleasant and cordial but it kills me inside. I told her i'm over it and act like I am. But i'm not.

Worst thing: our team gets together sometimes on weekends and she brings her new BF. This guy also seems to want to chat me up all the time as well (her suggestion?) This the worst.

How do I deal with this? Any similar experiences you've had would be helpful.