Why do I?

Why do I let my son choosing to be with my ex make me so hurt?  I invited him to my Halloween party a month ago and he never let me know if he was coming or not.  Then tonight I spoke with him and he was telling me about the party up at his dad's.  I remember all the parties we held,  I guess the prospect of a hot tub and a bonfire out weights what I have to offer.  I also hear that my ex sister in laws were there.  Why do I let myself keep getting kicked by these emotions.  I guess I thought they were my family too.  I had been coping pretty well with the changes lately ad this just wants to make me sit and cry.  I think part of it is that I want the ex to be having an awful life and he isn't getting with my plan.  That is selfish of me but it is how I feel.