I used to be a text book 'nice guy'
What do you mean by "used to be" nice?
I'm telling you that is over for me because nice guys get screwed over.
I understand your resentment towards others treatment of you, but deciding to give up your kind nature is wrong-headed thinking. Embrace your kinder side, and just don't let yourself be taken advantage of.
I'm done with the nice guy persona. It's brought me nothing but pain and suffering.
People deserve to be treated nicely and you giving up your kindness is a retrograde move. Let your kindness be it's own reward and be happy in that instead of looking to others to make you happy.