Oh my, jo, you've got a lot on your plate right now. So sorry you're going through this.
You've got to somehow train your thoughts not to think about this concept of him "upgrading" and being happy ever after with the new girlfriend. It's simply not TRUE. He is not upgrading and the new girlfriend is actually downgrading. Remind yourself of all the reasons he needs to be your ex when your thoughts start to stray in areas that are not good for you. He can be someone else's "problem", not yours.
It's like human nature to compare ourselves to others in terms of material wealth, money, finances, age, looks, whatever, but we've got to fight that urge as it really does us no good. You are a wonderful person in your own right. Believe that!