I'm going to demand my ex-girlfriend stop seeing the person she cheated on me with.
Why would you demand your ex-girlfriend stop seeing the person she cheated on you with?
Because of the way she betrayed me, I don't feel like she deserves to be happy.
The relationship is over, you have no right to tell your ex what to do. It sounds like you are still dwelling on the past and that is detrimental to your mental health. It's time for you to move on and find somebody else.
It's unfair that she gets to be happy in a new relationship when I'm so miserable.
I know this was a very difficult break up for you but you have to forget about the past or you'll never be happy. Try to sign up for some online dating sites and see if you can meet someone new.