See a counselor -- know the issues.
This sounds like a pretty tough situation, and I'm not sure I understand all the ins and outs.
When Does She Have Time For The Angst?
I know you say how wonderful you are and how great the relationship is, but what I'm coming up against is that as busy as she is, raising kids, attaining career goals, spending time with you, when does she have time for all this extra angst?
What the heck does she feel she is missing? How could she have time to form a relationship with someone else if she leaves you? Hell, most of my friends don't have time to do much else after working their 65 hour weeks -- they would go home and thank the good lord for a special partner who was as supportive as you describe yourself.
What Are The Real Issues?
So what I'm trying to say is, make sure that you know the issues involved here. I think you should go to a counselor, either with her or without her, to resolve the hurts that you are feeling, and to see if there are other issues.
Also, if it were me, at this point I'd sure be trying look out for my own interests. And your children's interests. But it sounds like a tough situation.