Why am i put on the back seat for the ex

My partner of 18 months had to go away last week to her daughters graduation she told me there was not enough tickets so basically i count go so off she whent met up with all her other children (all adults now) and her ex hubby (seperated 7 years) they then went for a nice dinner afterwards then next day they all met up again and shared presents and had xmas dinner together playing happy families all the time im left at home i feel like im left on the back seat every time the ex turns up this isn't the first time its happened, i feel i should of been invited even if they couldn't get me a ticket and the kids should of shared half day with dad and hald day with mum and me as i am her partner now of 18months, am i being over sensitive or should i say enough is enough if you want to exclude me every time he comes to town then im off ,i dont have a problem being in a room with him but apparently he would with me i believe if she really loved me she would stand up fopr me and not him and say no this inst happening me and my partner arent being separated just cause he doesn't like it