I am in a good relationship with a woman. We met while we were still married to others and then we got divorced. She has two small children, I have three who I cannot see.
She has started to get on with her ex-husband after two years of acrimony. That's fine, but yesterday she went into town with him and their kids and today I'm expected to sit with her, him and his new girlfirend.
I feel like a complete spare part, as if they have their family unit of ten years that I can't enter.
I've talked to her about it but we ohnly end up arguing that Im being selfish. Am I? Or am I becoming a cuckold?

You are there for moral support for your girl in an awkward situation, and yes its very selfish to make this about you. She is probably not looking forward to this either. Just go, and make the best of it, and stop whining.