First get proof so you are certain. Then you have to process those feelings, don't stuff them or they will come out at the wrong time. Is a affair a deal-breaker for you? How does it make you feel? The DBing tactics are to first bust the affair then work on busting the divorce. Realize you might not succeed on either of these, it's not all up to you.
Do you have access to her e-mail, phone records, and banking activity?
yes, you will let her know that you know about the OM and that her behavior is unacceptable. You will bring it up in the form of a boundary and it will be unreasonable to her, expect it.
"Wife, I have decided I will not share you with another man. If you do not stop contact with him, I will ____________ (pack your bags, divorce you etc.) To ensure this we will have complete transparency, I will have access to your e-mails and phone records."
Once you have proof is when set the boundary. Turn off her cell phone, stop the internet access, you pay the bills directly.
M22,H45,W45 S21/18D12 Retain faith that you will prevail in the end, regardless of the difficulties and at the same time confront the most brutal facts of your current reality, whatever they might be.