Letting it go doesnt mean have a new boyfriend. Eventually you will let it go, and if you really cant you will live with it. There is no magic answer. I guess the "new girl" part bothers you more , and more you think on it , longer it stays in your mind. You can not let it go without taking your mind off it. Give you attention to something else if you can. If you can not, then just live with it.
p.s: Nothing is wrong with you. What you are doing is a very natural human behaviour though its not healthy in the long term.
“Do not judge, or you too will be judged. For in the same way you judge others, you will be judged, and with the measure you use, it will be measured to you.
“Why do you look at the speck of sawdust in your brother's eye and pay no attention to the plank in your own eye?
How can you say to your brother, ‘Let me take the speck out of your eye,’ when all the time there is a plank in your own eye?