I'm married for 29yrs. to the same person, and when our problem occurred Oct. 1999, was a too close relationship with his female co-worker, I always trusted both, and never had mistrust when they had to go overseas,etc. on business.
It is when I was told and handed to me by him, a copy of his itinerary for his Calif. trip, and he quickly grabbed it and gave me a different one, the originally handed to me one was hers, she was not going with him on business but a different place in CA for a course, they had it all planned he picked her up at her hotel and would wine and dine, one place on San Francisco Bay, I still didn't know till I found a charge acct. bill, accidently in a bag that held a "guilt gift" that I had then questioned him. See, when a spouse starts lieing or hiding stuff, that is suspicious, if he was innocent, he would of just told me all about it, I've always been open, never felt suspicious to this the regular travels for business, and never confronted him, I truly always had trust. There was another time too, he claimed to be with the group from work, turned out just them, duh? I'm sorry, when a spouse hides things like this then the so-called friendship has evolved into a "relationship". BTW, she was married too, and her husband had a twin, his wife said she was fooling around with him, got caught in bed and said she thought it was her own husband, yeah, right. She had other things about her, I won't get into.
We did work things out, took time, for we had a long marriage and were going steady for almost 7yrs (since teens) before we got married, we were not new to each other.
We can forgive but not always can forget.

(((((((( MP )))))))) I wish you well.

BTW
I do all our bills,and record keeping/filing,and not because of this event(s), it's always been an assigned "job" as the wife,ha!ha! Well I saw a few charges that didn't relate to the "guilt gifts" that he gave to me, but to her, even when I trusted them before that devastating day, we'd pick out gifts together for her and another tramp he worked with, again I never realized how deep things were getting, I was so naive and downright stupid. After that I requested copies of his business account statements from AmEX, his company paid all expenses,and wow, not only do they pay or paid for the fancy hotels and fancy meals, they didn't realize they paid for some trinkets for the slut, when I played "detective" and called the stores on the reciepts and asked what the items were, etc. Anyone that didn't see this whole thing as an affair or spouse being untrustworthy has to be a real fool, and I no longer was going to be one.
My main point, and my therapist is the one that reinforced my thought, "when a spouse starts sneaking stuff" then it is as good or the closest thing to an affair, hubby even saw the light when that was presented during our marriage counseling. Counseling can help sometimes, I'm not against it or making ammends.
I just had said that if this ever happened again, I would leave, I refuse to be made a fool of.