Someone who can’t stop talking about their ex is still in a relationship. Just not with you.

Occupied attention is occupied whether the feeling is affection or resentment. Constant reference to a past relationship means the processing is still happening, and there is not much room for a new person while that work is underway. It is rarely a deliberate insult and it is still a real obstacle.

Notice how much of the conversation keeps returning there, over a few weeks rather than one evening. You can say plainly that it comes up a lot, without accusation, and see what happens. And be honest about it in yourself, since starting something new is not a way of finishing something old.