Two people are guilty in betrayal: he paid little attention to you, and that's the result.
To forgive betrayal or not is his business and only his decision. Then it is very difficult to restore trust.
Now your ex wants to be alone, and you have to pack up and leave him, give him time.
Maybe a week or two or a month and then try again to establish a relationship - first by talking. And you will see it by yourself. From my experience: it is difficult to forgive betrayal, and it is even more difficult to live with this person afterwards and every time he/she goes with friends to a movie / restaurant or somewhere else, to be afraid that the situation will repeat.
Good luck to you!