Why is my colleague treating me so horribly?

She is very demanding and tell me to do stuff and even the manager doesn't ask me to do a lot. She will make me look bad and stupid infront of the manager. Everything I do she criticizes, neverr says anything good. I always say nice things to her but sometimes she's unbearable. I was taught by the other coworker so I follow his way of doing things and she complains. There's also this guy who has a crush on her, when I finally lash out cuz I can't take anymore of her shit, he stands up for her. They are all friends even the manager. The manager is blind and can't see that she's trying to make me look bad. The manager will sometimes critique about the things I make for no reason when it's perfectly fine. This girl will also be rude to me and take her stress out on me for no reason. Please help.
Why is my colleague treating me so horribly?
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