It's hard to know what else to say, as unfortunately, it's your mess to sort out but I wasn't trying to give you a "bleak" assessment (even though it's a bleak situation, right?), just trying to make you see that you can shape the outcome the way you want it to be. From what I have read, you haven't done anything wrong and somehow taking "blame" for not making her feel special/loved/attended to or whatever is just nonsense. She could, at anytime, have said "Jack, some things are not working for me here and I need you to do some things differently", or would that have been just too easy?
And don't discount the possibility that more has actually gone on than she has told you about. Why would you believe that it's purely been restricted to texting? Because that's what she's told you? In my experience the first few councelling sessions will tell you a lot. Not just about how you feel but how things are going to turn out. If she embraces the whole thing and engages with the sessions, that's a good sign. If you just get a bitter outpouring of everything that's wrong with your marriage, that's not so good. And don't think that deleting his contact will stop her (if she wanted to). She will have memorised the number.
All I am saying is get prepared (someone has already said that in the other thread).
I would also ask her the following, in a calm way obviously - clear the room of all soft furnishings first 
Does she realise that even though "John" might be making her feel special and loved and wanted, in fact most likely HE just wants quick shag or two? After which he will disappear and leave you and her to pick up the wreckage. How does she feel about that?
Also, if John is so great, how come she isn't marrried to him and bringing up his children right now? Something must have gone wrong between them at some point. What was that? That he didn't make her feeel "special" enough and she ditched him for someone else? Something else?
Sorry if you feel like it's a bit of a bleak assessment but it seems to be a bad situation but as a lot of people have said, the best thing you can do is to decide what YOU want out of your realtionship. If that fits with what she wants, then you have a chance.