Can u trust twice!?

ok , if u love someone , and this someone lied at me TWICE about another guy that i told her not to talk to, because he was flirting with her..., she denied that he was flirting untill i proved her to her, so i told her that we have to break up if she keep on talking to him. so she PROMISED me not to talk to him any more but she did, i told her , then she swore to me that she wont , and then i discovered that she was talking to him again, i told her about that , and she didnt talk to him anymore, so the problem is that i cant trust her again!? thats my right! am i wrong?! she gave me the p. to her facebook account but now i doubt that she talks to him on msn or whatever! i doubt in her so much to a point that it hurts me too much!! but i dont wont to talk with her about that bec. if somthin. like that happened again i will dump her so fast. but till now she is not doing anything wrong!!! but plzzzzz can anybody tell me what to do?! i wana have some comfort!!