Michele, you don't want to be a simpering idiot, but you do want to let him know that you do not agree with this seperation, will not COOPERATE with it, and are willing to correct the things in your marriage that made him unhappy. [if that is true, if you are not willing to meet his needs you must be honest about this]
You must sound like a BROKEN RECORD with those 3 points.
1. Do not agree with seperation
2. will not cooperate
3. are only willing to work on marriage
No wimpering, no whining, no lovebusters. Got that?
In the meantime, say nothing about the OW, but find out who she is. Hire a PI, put a GPS on his car, put a voice recorder in his car. YOU MUST FIND OUT WHO SHE IS. Subtly ask him the NAME of his new trainee and track her address. Track down her parents, etc in
www.peoplefinder.com. Don't TELL him you suspect an affair until you have all the information.
If you have her cell phone #, you may be able to get her name from intellius.com via a reverse look up. A PI could more easily get ALL the information very quickly. But, you MUST quietly get this information if we are to be of any help to you here.
And let me explain why. The biggest threat to your marriage right is this affair. This is WHY your H is leaving you. Until that affair is busted up/ruined you will have no hope of meeting his needs in any successful measure. Your goal here is to interfere with this affair and hasten its death so you have a CHANCE.
The most successful way of ruining his affair is by exposing it. Affairs thrive on secrecy and when they are bbrought out into the open, it creates great conflict by ruining the FANTASY EFFECT. It forces the affairees to see how sleazy they look through the eyes of others. It causes them to DOUBT the wisdom of their actions. This is what you must do.
But, you cannot do this until you have all the FACTS. Getting these facts will help you win this battle. So, I don't care if you have to follow him yourself, you really have to find out what is going on here. AND QUICK.