Just out of interest everyone, what exactly is it in my story that suggests she's cheating or has someone on the horizon? Is that just the usual scenario or is there something specific you've read that makes you think that?
No one has a crystal ball or can tell the future. There are no easy buttons here either.
Normally a woman does not leave one relationship without having another one in mind.
That being said does that mean it is guaranteed - NO but very likely.
Sometimes the relationship is a character in a book, a romance novel for instance. You may never be as good a the HERO in a romance novel. So with each scenario there are plus's and minuses. The bad news when their is no affair partner the crisis takes much longer to burn itself out. SO be careful what you wish for.
This is a marathon not a sprint, my suggestion is to take things one day at a time, one step at a time and learn as much as you can.
I have over 30 books read on these topics, and really feel I could teach a PHD class on some subjects.
You can learn too.
Best way to eat an elephant is one bite at a time.