Why was Elena such a b**** to Damon in VD season 3 ep. 14?

I think it was an act of impulse. After all Damon was trying to stop her from seeing Esther and trying to keep her safe. It irritated me how she always told him to care more and then she tells him to care less. But later on she tells Stefan "I said something I didn't mean." or something along those lines. She does regret what she said. It was in the heat of the moment.

I feel as if she is but isn't acting like Katherine. Katherine toyed with both of them and had her own fun. But Elena doesn't want to toy with them, she wants no one to get hurt and just choose one.

I think Elena was annoyed by Damon, because he always tries to stop her [which is of course good, since Elena's plans always fail] from making her own decisions. At that moment, she was just mad at him, so she impulsively said that. You could see she immediately regretted it. She's a human, 18 year old girl who makes mistakes like that. She now has to face the consequences of hurting Damon.