I understand the struggle. Even on the good days I sometimes find myself having negative thoughts and have to make a conscious effort to redirect them.
I have a mental image I use of me from one of my GAL activities (mud run) without spouse. If I find myself picturing W with OM, or remembering one of the emails they exchanged, or wondering if there have been any secret contacts, I switch to that mental image of me being awesome. Focus on remembering that day and that moment, and how it felt. Picturing the weather, and the post-race cheeseburger. Think about the future fun runs I am already looking forward to, etc.
It has been effective for me, and helps reinforce to myself that even if after all this we still don't make it... I will be ok, and it will be her loss.
OW isn't a threat to you. You're a strong lady and will make it through whatever happens, so don't give her power over you.
Me, WW - Upper 30s BD - Apr 1 2016 EA - Apr 7 2016 (discovered; ongoing for months; did not confront right away) Confronted wife about EA - May 17 Wife sent NC email to OM - July 11