Sounds like the business has outgrown her. I read someplace that a lot of small business owners have that problem when they become sucessful. They just cant give up control. They feel like they have to oversee everything or it wont get done.
You need to persuade her to hire an office manager or a secretary or whatever to take some of the load off. From the sounds of things she needs help. Thats why shes stressed. And not from you. If youre working 12 hour days then youre already doing your part.