Ah man that litereally nearly made me cry the way you handled that msn convo. You truly are the better man Jim. Im so sorry this happened if it happened to me I would be horribly fucked up. Lifes complex (that doesnt excuse what she did though she is definatly in the wrong) and I think you should stay with her. This once a cheater always a cheater is rubbish. Give her time to come to you and talk again. What a rough couple of months for you.
Hope you have a great time tonight you deserve it.