First off stop pushing her into a corner, she is getting defensive.  Try to talk to her and explain how you feel do not expect her to answer and do not make her.. leave it open. Tell her you just want to explain how you feel and when she has had time to think it over you will be waiting with open ears.  Secondly, if she has sought attention from another, WHY? I am guessing that she is not receiving what she needs from you.