This story has red flags all over. Definitely on her end but also as you admitted to on your end as well.
She is having an emotional affair and yes this likely hurts just as bad as physical cheating. If it hasn't already taken place that is definitely on deck as well.
She indeed needs to stop with these funny games because it is ruining your marriage.
Not foreign to relationships and a lot of couples go through this.
Love is not always in a perfect homeostasis. It is up and down at times when life is hectic and the family faces tragedy. I think what this situation reveals is that something is definitely missing from the relationship.
In order to fix this, you first need to stop obsessing over this other guy for the time being and realize that she is finding in him what she used to find in you.
You have to go back to the beginning and re-discover what it was that made you two fall in love with one another. I strongly recommend focusing on discerning those things as well as finding a new edge on life that the two of you can both envision and enjoy together. Really concentrate on doing that and fight fire with fire buddy.
Love's everlasting capacity is exemplified by being able to rebound from confrontations with life's challenges and adversity and re-form that bond while falling in love all over again.
In order to move on you need to accept that this affair has already happened, not dwell on it any further and recognize that this relationship was partially broken which is what caused it to happen in the first place. That's why she went astray. The more you play detective with her phone and computer the more you break the relationship core values and entice the feelings of the emotional affair for the other two parties.
I get it though. Right now you are stuck on this. You have to shift it out and stop being the whiny male about it. She wants a bold presence with testosterone flowing through it and that needs to be you. In order to be that person, you need to focus on strengthening yourself and drop off the emotional baggage you are carrying with you now. It's like a bunch of black clouds ready to crack. You've got to gleam through with the sunlight.
If she cheats, it is not your fault. Plus, the statistics would show that she will regret it immediately. She is attracted to the idea of feeling something new and not necessarily to him and definitely not unattracted to you. As I said, you just need to focus on (1) you (2) making her attracted to you again and (3) reforming that bond not by focusing on where it got lost but using the foundation of the relationship history to discover something new you two can enjoy together or else the relationship is a losing battle.