Every marriage has its challenges. Some, even very major ones. NONE of it justifies her breaking her marriage vows and having an affair. She should have sat you down and said "This is serious, and I don't like how I'm feeling. I'm beginning to feel attracted to someone else, and either we get some marriage counseling NOW, or I think we may be in trouble."
Or some such.
In all likelihood, she was probably already in at least an EA with this guy before that December talk.
