Don't blame yourself or let her blame you for the A. That was all her choosing. She could have done a number of other things to fix the M before that. So no matter how bad a H you may have been the A is not on you.
Are you supporting her in any way? If so stop that. Financially, emotionally, etc. If she is with an OM then you don't need to be a part of that. She chose him to take your place. Let him have it all.
A's typically don't last long. 4-6 months before the shiny newness wears off and all the warts start showing her that the OM isn't that special after all.
Work on improving yourself. Physically, mentally, emotionally. GAL is for you to be the guy only a fool would leave.
Always remember ACTIONS are strong WORDS are weak. Be a man of action. Don't tell her what you are going to do. Just do it. Especially don't tell her you are going to do something just to get a reaction. Super weak. You gave her a month. Have you decided what will happen in a month if she doesn't come back? It needs to be an action, not words. "if you don't come back in a month I'm going to ..." Weak. Just leave it at "If you don't move back in a month I will have decisions to make" Leave it vague and just act when the time comes.
If you aren't OK with her living with OM what have you done to express that? Just words or did you take an action. Like just stop doing something (texting, calls, money, help, etc).
Me:49 W:45 M:19 T:22 EA confirmed and ended 8/2014 S:19,17 D:9,5