I was gonna say the same as Maz. Get the ex to take them for a night. Or go spend a night at a mates house with the kids. Preferably someone with kids, who you can talk to. That way your older ones will hopefully go off & play to give you a bit of time.
I don't really know what else to say, just hang in there. Remember that we are all here for you. It will get easier. It might take a while, but it will.
I hope you feel better soon.