What this means is that you can’t evaluate yourself and how you feel based off what he is doing with someone else.
Unless he is a spiteful git, he hasn’t moved on to spite you, he’s moved on because he’s getting on with his life.
Whilst you don’t need to do a cartwheel, true closure and the ability to leave your ex in the past, means that we need to either find a way to be happy for them regardless of who they’re with, or just not care.
I’ve often found myself in the latter corner and you tend to find yourself there when your ex has gone beyond the point of no return, there’s closure, and you recognise that you’re actually better off without him.
A little bit of bruised ego is natural but if you become consumed by his new relationship or exploits, it may be time to ask whether you’re really over him, and if you are; what is it about your own life that makes this news so unbearable.