This story is waaaay too familiar. I know what you are going through.
I lived with my boyfriend for 3 years and I broke up with him because he had some major issues with anger, marijuana, and alcohol. I had given him an ultimatum and acted on it. I know that he was still in love with me and it broke his heart. I was hurt too but I knew it was for the best.
Two months later after moving out of our apartment, he moves back into my complex...with a new girlfriend! We were 25 and his new girlfriend was 18! It was like he was throwing it in my face that he had moved on with his life and wanted me to hurt as much as he hurt.
That took me a while to get over and I am happy that I did not look back and succumb to begging for him to be my lover again.
Fortunately, I had others to talk sense into me and tell me that he had issues and was trying to hurt me. If he really loved me and had some sense in him, he would have gotten help and come back to me. And yes,...I waited. I hurt so bad for a couple of years after that experience that it took me some time to get back into looking for guys to date.
My brothers told me that he was trying to hurt me, but also in a strange way, was trying to fill the void of the relationship that he had once had with me. He needed a girl to come home from work to and the familiarity of the apartment complex to remind him that he is not out on his own. It made sense to me and sort of helped me get through the rough time.
If I were in your shoes, as I was 2.5 years ago, I would move on. He is not serious enough to waste more tears on. He sounds emotionally immature. What about this other girl, why is she putting up with him? That guy is offending both of you.
Best of luck to you. Keep your chin up sweetie!