It's a hard thing to accept. I disagree though. People are allowed to feel hurt, vulnerable and unhappy. If you are in the place where you say "My ex just broke up with me, but I'm fine, she's doing better" you certainly are a virtuous person.
But, we aren't Buddha. We're human, and we get hurt. Physically and emotionally. What people tend to forget is that it is ok to feel hurt. That it is natural for a human being to want to smash in the head of the guy who is shagging your ex. To be disgusted at the thought.
As a (straight) human male, you are biologically programmed to reproduce. And therefor, you want that to happen with your sperm. When somebody else is with your ex, it isn't your sperm, and that is disappointing. From a biological point of view. I think we keep forgetting what we are (an intermediate species, but we won't talk about that now): human.
There's a lot of talk about how you should behave. How you should feel. Less talk about what you do feel, and why you feel it.