Oh you poor thing, how difficult. My heart goes out to you, and even more to your children...
"I know my 13 year old son acts out to his younger brother and when I try to talk to him he just gets angry and says his dad doesn't have any right to treat him that way either but he does so I think my son feels justified in his actions toward his brother." And your young daughter, in tears over her dad. And what about the littlest one (7?) God love him.
As you said, it must be really hard to follow through in the difficult step of leaving. Do you have to make that decision right now? Maybe now is the time , for your and your kids' sake, to collect info, research into what all your options realistically are, then you will hopefully be better equipped to make the right decision for all of you, especially you and your children.
I've not been myself, but am thinking about (my AH died recently) going to AL ANON, as people here speak so highly of it. Could you try that?
You sound like a very caring mum, and your heart must be breaking, knowing your kids are suffering.