Can anyone help me make sense of this please?

Before you even try breaking his heart, you need to set a time where you can talk to him, when he is in a good mood, because if he is in a bad mood then his reaction will just be worse. I know it's hard to tell him these things but you can't go on living thinking that you made a mistake if you leave him for this guy, if you do that is.. And well tell him straight out about what you are thinking, just not about his friend just yet. Let him know you need him to change and if you don't see his progress if he does decide to change then you are there to do what you think is right. And if leaving him for his friend makes you happy, then do it because you cannot always be watching out for his happiness, you come first.