Hello Bruce, I'm really sorry to learn of this and there must have been a reason why you looked into her messenger account, otherwise, there would be no need.
Can I ask you what would be worse knowing that she could be going to have an affair or looking to her account, well, if she has nothing to hide, then maybe slightly annoyed, but if she's going to cheat, then she will be embarrassed and caught up in a situation she needs to explain?
If I was in your position I would do exactly the same, it's something you want to know and if your relationship can eventually be repaired.
If a spouse/partner is going to cheat on you, then there is someone else trying to attract them for a certain reason and doing this only feels exciting as two people bond with each other and fall for one another, but it can't last.
This is not an excuse for this to happen and maybe your relationship needs an injection of excitement, but I'd really like to hear back from you.