Thanks for writing to Just Answer.
It seems there are 2 problems here: your recurring thoughts of her being with other men, and her statement that you do not give her a challenge.
I'll address the second one first: Of course, the best way to get an answer is to ask her directly what she means by a challenge, but I'll give you my take on it. She may be missing the flirtatiousness of early dating, where neither one is sure of the other's feelings, and others may be competing for the person. It's unrealistic of her to expect that in the day to day world of marriage.
If she needs more stimulation in her life, it is her job to find it by learning a new skill, going back to school, etc.
But to keep the spark alive, (and this is harder if there are children involved) you could build a little intrigue into your lives by planning a surprise weekend away once in a while. Make all the arrangements and tell her only a few hours before to pack her bags. Anything you can do to break the mundane routine of marriage will help.
As to your recurring thoughts of her being with others....
It would help you to "re-frame" those thoughts into ones that serve you, rather than hurt you.
For example, you could consider that your wife is attractive enough to get another man's attention--but she's choosing to be with you. Some men even use the thoughts to fuel erotic mental fantasies...and this helps them reconnect with their wives sexually, which in turn improves the connection between the two of you. Basically, you make the hard choice to think about those thoughts differently.
Think how much she must love you if she is willing to walk away from the excitement of an affair to be with you.
Women often have affairs as an attempt to recapture the feelings of being desirable, important, and worth pursuing that they had when they first met their husbands. Being taken for granted and not being noticed is very hard for some women. Work on bringing some of the things you did during courtship back into the relationship.
Please consider the two of you going to a marriage counselor. Pick one that you
are most comfortable with, as it is usually harder for men to open up in sessions. In the meantime, one of the best books I've found for improving marriages is How to Improve Your Marriage Without Talking About It.
(sorry I couldn't find an Australian Amazon equivalent) The book talks about the very different needs of men and women...one of the best explanations I've ever read.
This will be a process...there is no quick easy way to recover from betrayal.