I hope you can find some answers, but expect deflections and inconsistencies, not just because she is trying to deceived you, but because she does not know herself. Go with your intuitions and don't let deflections and excuses stand without at least a little questioning.
She thinks she can find her purpose and goal through this other man. Maybe she would feel so much more relieved, grateful, secure and happy if she could find it through you? I believe you have it in you to help her find this, but you've got to be sure that is what she wants, and somehow let her know you understand and are willing to help her. Make it empathic but not supplicating.
Your sitch is SOOOO much different from everyone here. I do not recall any mention of hard times, arguments, conflicting values, incompatible personalities - in short, there are no negatives. What seems to be lacking is a spark, the old excitement, the sense of life. Really, you are no different than any other couple except for one small fact. You have no kids. Any other couple on this board with as few problems as you would consider themselves to have an ideal marriage. The waning emotional bond between man and wife is bolstered by the connection with children.
Because you do not have that spark of youth, that sense of life that comes from hearing the pitter patter of little feet, your marriage seems to have gone dull, IMO. Not from too many negatives, but waning positives. Perhaps this is what your wife senses and is looking for.
Sell a message of hope and of a new future, her future, a future balanced between you two, where she can pursue her dreams, not a guilt ridden review of the past. Sell her something that will bring back that feeling of life and excitement, that sense of purpose. Sell her your excitement, here on this earth and in this plane.
