I'm upset that my close friend/ex uninvited me from his birthday party because he is seeing someone else
Why are you upset if you are no longer with them?
I still love him and it upsets me.
You should respect your you ex's new relationship. People are expected to move on with their lives after a relationship ends.
I'm going to stalk him and ruin the lives of those who are close to him.
I think that that mindset is not going to be very helpful for you in the long run. You should try to respect boundaries that you ex sets and not try to hurt them in new relationships.