Even if you don't think the marriage can be saved, it is worth booking some sessions with Relate or another counselling/
mediation service. It can help you both come to terms with what's gone on and make the break up easier to deal with.
The knock on for this is that dismantling the finances etc will be less acrimonious.
I would not rush to file papers but instead reflect on your situation and take,any action when the dust has settled rather than when you are distressed.
As to the situation you are in, different people manage their marriages in different ways. For some the earlier incidents would have been the end. You say you dealt with them and moved on. But perhaps your reaction to this latest "business" relationship shows that you didn't really deal with your feelings.
Your wife has had a life changing experience with her health as well as training that seems to have allowed her to really spread her wings. Is there perhaps an element of you feeling a bit rejected because she is more independent now the children are adults?
You talk about the support you have given her and that she is now describing you as controlling. Again this could be a function of her being able to have more freedom now the children are adults; her previous life may feel quite stifling by comparison. That wasnt your fault. It goes with the wife/mother territory, even in these relatively enlightened times.
Finally you mention concerns about messaging and having meetings with her business partner who happens to be male. Would you have the same concerns if the business partner was female? How would you feel if she was jealous of every female you worked with? Mixing business and pleasure is never a great idea.
Yes she has admitted to relationships outside of the marriage. I think rather than simply blaming her, you could consider what she was looking for in these flings/affairs. Perhaps she just enjoyed being seen as an attractive woman, not just mum or wife. Maybe she doesn't even know why herself.
It could be that her work is fulfilling that need in her now. It is great that she is realising her potential. You mention that your work has been stressful. I wonder perhaps if there is an element of resentment that she is enjoying an upsurge in her career perhaps at a time when yours is not going so well??
31 years is a lot to throw away without some careful thought and reflection. Life after divorce can be better but it can also be a lot worse for some. You are,splitting one home into two. Your children are dividing their time between you and you lose the person who probably knows you better than anyone. I would never urge anyone to stay in a marriage that was totally wrong but I would counsel you to think about what's driving both of you before pushing the nuclear button.
Hadenoughnow