He is showing you his true character. Believe him.
That may be harsh, but he's done you a favor. You are getting to see how he behaves, how he treats you and his "formers". I know it hurts, but let him go. His stuff with his EX is his stuff.
Do something nice for yourself. Treat yourself gently and try to get to the point where you see him for what he really is, not how you want him to be. Not with "if only", but the real, true reality of the situation. You deserve more than this. It hurts, but you'll get over it.