In my personal experiences I have learned that letting someone who cheated back into your life is a huge mistake! I have had guys cheat on me, swear they would never do it again, they were sorry, and all the lame excuses one could fathom. I had my heart torn out, ripped to shreds and stomped on more times than I would care to count, but it's always the same. Once someone has broke your trust, you can't get it back. Somewhere in the back of your head it's going to be there, and what she's doing just elevates it more. People change, sometimes for the worst. There is someone better out there, someone who will treat you the way you deserve, and respect you. Keep your head up and move on! I know it's hard, but it's the best advice I can give. Life goes on, sometimes relationships don't. You can't put yourself through the pain, and if she loved you, she wouldn't stray.