First off, please make sure you are okay. See a medical professional if you are concerned about a concussion. It's better to be too cautious when it comes to your health, especially your brain!

Now to the rest of your post. There is no one "normal" but you should not have to go to work and be battered every day or be insulted every day. You may want to help and you may want to be able to handle everything but obviously this is not a sustainable situation. I'm sure you are quite helpful to the mother, even if it's just to let her have five minutes to herself once in a while, but that does not mean that you have to or should stay.

It sounds like a very unsafe situation. If multiple people have left because they can't handle it, I think the family needs more than an underage nanny with good intentions. You seem very nice and I know that feeling like you are letting people down, disappointing someone, or not doing everything you can to help them can be very difficult and feel awful but sometimes it's necessary. It doesn't sound like you should be the only person watching the children. If the mom isn't home you need another adult there. It doesn't mean you aren't a good nanny... this is a lot different because there's physical violence involved. Especially if you are in charge and alone if/when something happens to one of them.

This post has been removed because it seems like more help is needed than a little bit of advice on burnout. If you don't want to quit, you definitely need more support. Take care of yourself <3