The new girl, why he is with her, how long, its all part of the same package. Things usually stay in our mind when they are unsolved. We ask ourselves questions but we do not get answers and those questions remain in our thoughts.
If you pay attention to it, if you can observe it, you might notice its not really about him or her but about yourself. Like, he was with her longer than he was with me, what is wrong with me? Why didnt he stay with me longer, am I not good enough? He didnt pay me the attention he is paying her, am I not worth it?
You see all these questions start like they are about the other person but actually indicating our worries about ourselves. And without facing them , they wont go away. There is a chance you can not still let him go because of these self questions . If that is true than him and her are just an excuse you project your self worries on. I can not say , yes thats it.. Only you can recognize if it might be true or not.
Be Well
Hermes