I am sorry you feel so troubled and although some have been a little harsh, try not to let it get to you. I think you are feeling very 'left out' and I do know how hard that can be, but as others have said, if he was going to go back with her then nothing you could do would stop it, but remember things were not right between them and whatever was not right will still not be right. So, take a deep breath and wish him well, remember he is doing it for the child and not for his ex. You will feel bad if you make a fuss because then you will make him torn between upsetting you or upsetting his child. So, just grit your teeth and get through it and be welcoming when he gets back, that is all you can do. There are going to be more days like this, school days, graduations etc. etc. and they will always share this bond, but that doesn't mean to say that you are not in his heart when he is with them.