I'm so sorry that you are going through this.
Firstly, you are not a fuck up, and everything is going to be ok.
As others have said, get the non molestation order, and get him out.
Some things which may help you:
Call the GP tomorrow morning, if they have no appointments, tell them you need an urgent appointment with the duty doctor. Get them to sign you off for a couple of weeks with stress or something. This will give you some time to get your ducks in a row and take care of yourself and the kids.
Contact citizens advice, they will be able to help with benefits advice.
Go to your mortgage account online. The mortgage charter states that you have the right to change your mortgage to interest only for 6 months. You can do it online, and don't need your exes input. This will reduce the payments for 6 months and take the pressure off.
Apply for universal credit online. You can also get a SMI loan from them which will pay some of the interest on your mortgage. You would need to repay it when the house is sold, but if there is not enough equity it is written off.
You can do this, and you are going to be fine. Your life is going to be so much better without that bastard, I promise x