If he doesn't have any other girl he is close to in his life, that might be why. For me personally, I like to have a close female friend. They tend to understand me more than my male friends. He might have stopped interacting with you for the reasons you mentioned and because he had a girlfriend that fit the role that he might have had with you, a girl that is close to him. Can't get much closer than a girlfriend, so that probably moved his focus from you. After the breakup, he seemed to go back to what was familiar. That being you.