It's sweet how everyone is acting here I think, the care being shown.
However to me it seems the OP is currently driving a bus, except she didn't know how to drive when she took over the wheel and is learning while dodging traffic. If she is at this current stage, 12 years into a relationship I must wonder what has been happening for those 12 years in regards to relationship skills...... it sounds like the married people haven't really been honest with each other for a long time, but stuck with each other because it was easier.
Now the new thing comes along and the man realizes he isn't really happy with the old situation. So following the path of dishonesty like he has for 12 years, he just continues to plow a new field whilst letting the old one slowly wither without much thought. Anyone interfering with his new field will pay a price because it's making him happy and he hasn't felt that in a while.
Tough situation, glad I'm not in it.