NAB, you are putting far to much pressure on yourself here. How long have you been on meds, what are you taking (i have terrible memory!).
The fact that you are posting your frustrations here means you are a great mum, so stop putting yourself down.
So WHAT if they bicker, i mean, maybe, and this is just a thought, it might now work, just leave them to it, perhaps by rising to it every time (i can totally understand why you feel you have to react) you should just make sure that no one is actually going to get killed or maimed and then walk away, they might just settle their differences themselves rather than relying on mummy to intervene.
So WHAT if they are not perfectly behaved little angles, i would be worried if they were.
I think one of the reasons you put this pressure on yourself is because, as you say, you were a great nanny. But it is different with your own children, "this time, its personal" This time if they behave badly you take it personally rather than just dealing with it. Can you take a step back, pretend you are the nanny and deal objectively with it, easier said than done i am sure.
I hate having shit days, but this will get better, i promise.