I have to echo Starsky's admonition about confronting without a plan.
That's not to say a "hit hard and dodge" plan, but a plan nonetheless. Do you think your w would agree at some point to attend Retrovaille? It's a marriage weekend for marriages in crisis. We got a lot out of it. There are also individual workshops for personal growth and addressing private individual issues that can affect marriages, and those include "Essential Experience", Life Spring and Imago. There are others but I have NOT heard good things about them.
Workshops tend to be a LOT better than most MC's are, b/c many mc's merely help you re=hash the past and validate whoever wants OUT of the m. The workshops are more intense and teach new behaviors a lot faster than one hour a week can do.
Realize she wants out of the marriage b/c she THINKS there are problems, and chances are, she thinks you are the source.
SOME of her beliefs might have validity so whatever complaints she has made in the past (eg., you are not attentive to the kids, or you don't give her the attention she needs, etc.) so you need to counter the negative images she has, or created, of you, with positive contrasting images.
IF she said "you're always late", you become Mr Punctual! This is what a 180 is and I believe you said you have read the Div Busting or Div Remedy books? Good...they really help and they're different than most other approaches.
Issue no ultimatum UNLESS you are fully prepared for any response she gives you. That means, assume the answer she gives is not the one you wanted...THEN decide if you can handle that.
When she finds out that she is one of many OWs for the trainer, she'll be even more humiliated and possibly even "heartbroken" - which won't be easy for you to see. (Don't be the deliverer of that message if you can avoid it. People Do like to shoot the messenger...)
Chances are, ironically, That might make it harder for her to come home, and you will need to do two seemingly contradictory things:
You'll want to "Keep the Road Home, Paved & Smooth", (b/c it really will be harder for her to come home, once it's "out" than you realize)
but you'll also have to be setting boundaries...not to punish her, but to protect youreslf and keep your own sanity/self respect.
Make sure whatever boundary you do eventually set (and enforce) is something that comes from a place of healthy self respectm and not a wounded ego or desire to "teach her a lesson". (That tends to backfire big time) You'll need a ton of support. So will she, in her own way.
There are many tools out here, so avail yourself of them and post often. People here care.
it's a great place to be, for a lousy reason. Good luck.
M: 57 H: 60 M: 35 yrs S30,D28,D19 H off to Alaska 2006 Recon 7/07- 8/08 *2016* X = "ALASKA 2.0" GROUND HOG DAY I File D 10/16 OW DIV 2/26/2018 X marries OW 5/2016