I agree with both of these, and yet I have to wonder -- if you've learned a lesson, does it really benefit you to keep learning it over and over again? Is it really "learning" if you already know it? At what point does it become gratuitous self-torture? I think with some people, depending on who you are and who they are, there's a point where the sane response is to just say to yourself, "Right, I've done what I can with this" -- echoing your sentiment that all you control is yourself -- accept that the situation isn't going to change, and exercise your option to stay away from it.
I agree with Mary. I got (and can get) a lot out of forcing myself to train with difficult people. Over the years, I have learned a great deal.. about myself, about them, about how we can possibly interact.
But I certainly don't have to keep doing it to make what I learned useful or applicable, and I would never allow it in my dojo unless I genuinely believed that it would improve the training. Even then, such a thing is highly situational and should be temporary.