5 years is an awfully long time ago.
If he's been faithful since, you should let it go.
If you're reading his stuff, it suggests you think he's doing it again, however the way you are going about it is wrong imo.
People are entitled to a private life, even if they are married and you shouldn't be going through his personal stuff.
The trouble is, we could say let it go and you might agree, but I don't think you will.
Have it out with him if you want, or forget it, but genuinely forget it.
Harbouring stuff like this when you don't want to could easily kill your marriage and he would never know why. I don't think you want that.