Strength to you, your sitch sounds very similar to mine for the past year and half. My husband was at home most of the time, but would visit OW on weekends. He's still here, but just recently started D paperwork. I admit that I snooped a few months ago and found a 'happy anniversary' card from the OW. OUCH! Such pain, and I just kicked myself. However, when H started really pushing for D soon after, it did help explain his motivation. I think mine is getting pressure from OW. I'm not sure if it was in any way worsened by my attitude of knowing it was their 'anniversary'.
Probably you are better off not knowing, and if your marriage is in stagnation, at least it isn't moving backwards, and there is hope.
Me - 38 Husband - 40 MLC! Together 12 years Married 11 years Still the love of my life Forever only lasted 10 years before his MLC and affair