I suggest that, if you go to the party, you busy yourself in the kitchen for as much of the time as possible. Cutting up the cake, wrapping and putting in the goodie bags can be helpful and takes ages if you try! I used to find this helps, and the rest of the time I took photographs of the children to keep myself occupied. Remember ex will probably also be apprehensive. You have as much right to be there as he does, so don't let him spoil your enjoyment of GD's special day.
My GCs are older now, so no birthday parties, and anyway exDH does not attend family gatherings since his second wife was rude to me after I greeted them politely at a christening, and then shouted abuse at me in the car park! I had the silly idea that we could be friends, as she met him a while after the divorce.
As I have to keep advising DD1, who is going through a dificult divorce, try to act with dignity and put on a brave face, and show him that he cannot hurt you any more. I hope to hear that you have survived and that it wasn't as bad as you feared. 
Is Mumsnet down today (13th May)
By special request, let’s discuss our favourite Classic Music and why?





