It's hard when you've begun to let your guard down to have to put it pack it. I've always felt like it takes more effort each time. But truly, you still have the inner stength that you need and it will be enough - because it has to be.
Share the burden with your husband. He will help you with the stength on the days/hours when yours is waning. Let him in and let him help. You do not always have to be stong hold... sometimes, you need to regroup so that you can be the lynchpin again - especially since you need to be that source of stability and strength for your girls.
Reach out to others... email, call when you need to unload and share with someone outside the situation.
Remember that it is your girls that are central to this issue - their physical safety and their emotional safety. Thinking of this will give you that inner strength. It's where I pull it from and it always seems to be there when I forget myself and think of my daughter.
Being proactive helps, too. As another poster said, record everything... it may not be preventative, but knowing that you have it down on paper can be helpful.