mybest, it may be hard to hear, but yeah, 99.9% (I made that up) of the time, this situation is an affair. It doesn't change how you react, though. You still need to get a life, maintain your integrity, and be the woman he'd be a fool to leave. Affair or no affair, this just makes you more attractive. If he sees that you'll just be there, no matter what, he won't feel the pain of losing you. He has to see what he'd be giving up.
One caveat to this. Some people see infidelity as a deal-breaker. mybest if you are in that camp, then your actions would go from DBing to filing for D.
If infidelity is not a deal-breaker, IE you still want to try for R despite infidelity then it doesn't really matter does it? Take NYGal's advice and move forward.
M(53), W(54),D(19) M-23, T-25 Bomb Drop - Dec.23, 2017 Ring and Piecing since March 2018