If I may, if you noticed in ch 13, JJ was willing to listen. Even before that, at the club, he was somewhat willing to talk. But the conversation cannot be him feeding Leo information again. Not Leo realizing what is wrong because he was told it was wrong but rather realize that his behavior was wrong by genuinely recognizing they were. Otherwise, it would feel superficial, just to get by and be able to get back to how they were. Leo's approaches on both occasions felt that way. He wasn't seeing JJ as a partner but an assistant or lucky charm he lost and wants to get back. I think JJ is feeling that so he's reacts as if he touched a hot surface whenever Leo talks to him.
Even at the hotel. Leo is not facing the reality that they've broken up. He again fronts the 'me' statement, "why did you not tell me?" sounding like the world revolves around him, rather JJ's world revolves around him.
A third person has to step up and tell Leo what is happening because his perspective seems like it's in a bubble. Leo can always ask someone else to help him analyze but he's not doing that.
Listen I get JJ, I can understand where he’s coming from. Leo’s immaturity greatly affected their relationship, the fact that it took a breakup for him to realize hurts.
But at the same time, JJ could also just sit Leo down and have a serious conversation with him, Leo may be dense but he isn’t dumb enough to not realize his own mistakes (Even if it took him awhile) couples therapy is a real thing that helped people maybe they should’ve tried that