First of all, you have to accept the fact that your ex is with someone new.
Scaring/harassing the new girl away is not going to help.
You don’t want him to feel he is cheating on his girlfriend.
So, make sure you are just asking him to “catch up” or ” go for a coffee”.
(Unless you just want sex, and he is up for cheating.)Usually, during this process, he will decide to leave his new girlfriend and come back to you.
But if he doesn’t, you have to ask him about it some time.
He just went from one short term relationship to another.
In this case, I’d generally recommend you to move on from this short term relationship. Do you think if you and your ex got back together, you two will be able build a relationship with strong foundation? Did he decide to just leave because he was not satisfied or he discussed his problems with you?
If he brings her up (which he probably won’t), change the topic casually. At least not until you are 100% sure he wants to be with you. You don’t want him to think that you are only interested in sex.
Let him know that you want a real relationship and for that, he will have to commit.
I know it’s a tough thing to do, but if you did everything right till now, then I am pretty sure he is going to choose you.
When it is time for a dating relationship to end, they end it and move on.
That’s because being friends with him while he stays with his new girlfriend is not good for your emotional and mental health.