There is nothing left to lose by returning the insults. Well, I have tried that and since they already took from me what they wanted, I still lost out when I fought back with words.

Insulting people who have bullied me and manipulated me has had no effect on them at all. It's affected me though. It makes me feel even worse because what's the point of insulting someone who just destroyed an opportunity for me with their lies and deception? They could care less if I try to establish boundaries with them after-the-fact. So, I understand what you're saying, but I guess it doesn't seem worth-while for me, to argue with someone who won't respect me or who won't check-themselves for their own bad behavior if I point it out to them. I have tried. With no luck.