Most Helpful Girl
You say you've not talked for a month, but how long ago did you break up? Did you call him and ask him to say that when you found out he had a girlfriend?
I doubt that he is just using her. He probably has moved on and hence no longer has a need for you in his life. I've been through something similar with my ex. We broke up because of distance and remained in touch for over a year. I've always felt that as soon as he gets a new girlfriend, he'll just stop talking to me; I voiced my concerns to him and he kept telling me that we'd still be friends even if he starts a new relationship. Well, one day he suddenly stopped responding to my emails and about two months later I found out he had a girlfriend. I have not heard from him in almost half a year now, and I have no urge to message him and I am pretty sure I will never hear from him again. It hurt a lot in the beginning because it showed me that my friendship must have not meant anything to him.
As for how can he leave after everything you went through, I don't know. But people can, and they do. I know it makes you think that he must have never cared, but that's not the case. There was a point when he cared about you dearly, and maybe he still does a little. But he has started a new chapter in his life, and the best thing you can do is do the same.