The concept that helps me most about difficult, unfair people, specially close ones is the ancestors line’s history and the person’s own childhood. A person who had been loved and nurtured in infancy usually turns into a well balanced and loving person? if you look at their own family and see lack of love or stunted emotional response there will be blockages and maybe a copycat horrible behaviour. The struggle as you said Miranda, is when you try to change, understand and accept and the other person doesn’t care or seems almost to take advantage of your goodwill? If in a vicious loop of Tit forTat or BlaBlaBla with nasty exchanges or tired conditioned responses it is truly draining…You can only skilfully avoid the bait of toxic exchanges and walk away!