Do not chase him. You are now seeing the real person he is, the old him was an illusion. You are chasing a ghost.  Believe what he says.. Focus on being the best person you can be.  Go to therapy to help focus on why you want someone back who treats you disrespectfully.  Do not think that you have have the power to change him, if he wanted the relationship he would have stayed and worked on it. 

It is confusing when an intense relationship freezes over, we often blame ourselves when the euphoria dips then peters out all together.  Then we lower our standards and accept things in an attempt to keep the ship sailing rather then accepting that we cannot change another. You did nothing wrong, you just became familiar and routine to him and he is now seeking the rush from something/one else.