Hi, Apple23, welcome to our little club that nobody wants to join. You seem like a decent chap at the core (see what I did there?). I can bet your mind is reeling at the moment. Understand a few things, going into it.
She knew it was wrong. You say she says that. So there's not disputing in her mind that this wasn't acceptable behavior.
She planned on accelerating the behavior to a physical confrontation. She was an active party in those discussions, she doesn't deny any of it, she had intentions to be in a physically sexual relationship with this guy.
With that said, you seem to be thinking this was a "mistake" on her part.. Alll a biggg mistake, sorry honey? That kind of thing. Let's nip that in the bud. You are going to see this a lot on here. Mistakes involve a lesser degree of free will than intentions. A mistake might be spelling a word wrong, or painting over an outlet or something. A mistake involves doing something you didn't intend to do, or doing it wrong. By that (very loose, I admit) definition, Adultery is NOT a mistake. It is a deliberate act of will, one that is chosen. It is a series of active decisions.. usually starting from the very small and innocuous decisions ("Should I link to this old college acquaintance on LinkedIN? Y/N" then "Should I post a smiley face at this risque comment Y/N" then "Should I laugh at this even risquier comment? Y/N" then "Should I say something back flirtations? Y/N" then "Should I respond to his graphic suggestion or sexual innuendo? Y/N" You see how that works? Every step of the way, it went from the very innocent and deniable activities to something a little less acceptable, then even less acceptable.. then incredibly inappropriate for a married woman to be engaging in. The common factor? At any moment, your wife could have said "Hey wait a minute, I'm married!! This is WRONG!"
Well, she didn't do that. Instead, she collaborated with this fantasy to the point where she was more than willing to destroy your marriage to go sample a little strange (I'll be delicate here,you've had a rough couple weeks). You, your feelings, your children (if you have any) suddenly didn't matter at all to her, or she conveniently forgot them. So she could go sample some strange.
Does this inform your opinion at all? Because it really ought to. For me, adultery was always about the decision to act and continue to act, and disregard for boundaries. A partner that is willing to do that has checked out of the marriage. Your wife planned on doing all those things, AND staying in the marriage. You should ask her what her plan was for the marriage going forward, in a state of adultery. Was she ever going to inform you (doubt it)? Was she feeling the same towards you as before? What would change? She has sent you a clear message here-- your feelings and to a greater extent, the marriage itself, are an elastic concept. They are not a hard boundary for her. You say she is "incredibly apologetic". What is that except words? Do you want to stay in this relationship with her? What will it take to make YOU feel safe again? Has she even worked on that?
To answer your questions:
how can I trust her again? She was my everything and had 110% trust in everything before. Can I let this 1 time affect how I have felt about someone I have been with for so long and created what seemed like such a great life together.
You really can't trust her again. At least, not for several years. You are going to dwell on this for a very, very long time, and it will suck. You will be triggered by her betrayal. You will have nightmares, loss of sleep, lost of intimacy. It's going to happen. These are the consequences.
- am I dwelling to much on the “what if I didn’t catch that message and put a stop to it?”
Hell no, you're not. The mere fact that you DID catch her in the act is indicative that it WOULD have been a physical act. ONE SHE CHOSE despite the effect it would undoubtedly have on you and your marriage if discovered. She didn't even think twice! (I assume so anyway). You're luck you did catch her this early.
- why was she willing to have sexually explicit chats/send nudes to him, when she won’t even do that with me?
Well, in a word, maybe disrespect or disregard for her husband? Maybe. I don't know either of you. Was that ever part of your marital repertoire before? Have you asked her about this directly? How come a stranger gets to see what you deny me, your husband? It would be interesting what she says.
Personally, I think you are on very dicey ground here. You went to couples counseling. I advise against this, but you may both need to go to individualized counseling. THe marriage looked healthy to me, from your description. It didn't fail.. SHE failed. This is on her alone-- she needs to fix herself FIRST before recommitting to this marriage. YOU need to deal with infidelity trauma, which is another thing altogether.
Best of luck, man.. I know you're in the confused, what the hell just happened phase. You will soon be in the mind-numbing rage phase (Oh yes, you will. trust the internet strangers on this). Don't let that anger overwhelm you. You don't have to hate her-- but I would disengage and get some space for a while. Maybe she should move out for a while and stay with her parents while she sees a counselor. If you trust her to behave. Just a thought, you have a lot on your plate to parse and she might be triggering you. Come rant on here ANY time and we'll do our best to help.