At some point, the other person has to make a choice to forgive. Until he does, no amount of trying to prove anything to him will be good enough.
Podunk seems to like this idea of being this kind of fatherly guide to you, the sage one who will encourage and direct you toward what he believes is good for your personal growth. Perhaps it was all too much for him to see you find your own way and make your own choices (and mistakes), however insecure and self-doubting you were. He certainly seems focused on how you have "done everything all wrong" and that it is absolutely irreparable.
Also, I have to say that, despite what he says about wanting poly, I keep having this sense that Podunk really prefers swinging. Perhaps the possibility of your having a deeper emotional connection with someone else, as in polyamory, hit him harder than he expected and feels like a threat to him in some way.