I'm sorry you are having a rough time. I think it can be really empowering to force yourself to detach and go for lunch with the mindset of "I am totally awesome-- self-reflective and growing in a positive way. He sux and wait, did I mention how awesome I am? I am going to show him, my kids, and the rest of the world how strong I am. And then, later, when he is gone, I will have a good cry if I need to. Or treat myself to a manicure because I am so awesome."
I went on a vacation with my H and his family. Let's just say that I wasn't the happiest person at the happiest place on earth, but I put on a brave face for myself as much as for my daughter and everyone else, and let myself feel however I needed to feel in private. (That involved escaping to the ladies room to have a good cry a couple of times during dinner, if you must know the truth!)
