2 things:

1) usually people like this feel super vulnerable for some reason, so they protect themselves by going on the offensive. if you can figure out what makes them squirm / what they're sensitive about, you can use that to your advantage (and not in a shitty way, just to protect yourself, or else you'll turn into them)

2) don't let people take advantage of you. even other really nice people will without even realizing, so you have to be assertive. It can feel like being too aggressive at first, but remember that enforcing your own boundaries, rules that you play by and that you expect others to play by as well, is always justified.

do those and you won't have to worry as much about that shit getting to you. you start to see it as nothing personal, just a shitty person being shitty, and while you won't like it, you won't think about it. kind of like bad driving - once you start driving defensively as a habit, you're less annoyed and more prepared when people do something shitty.